My invoice says 1.5% per month. The contract says nothing about late fees. Can I charge it?

Printing a rate on an invoice does not by itself create an obligation to pay it, because an invoice is generally read as a demand for payment under an agreement that already exists rather than as a fresh offer of terms. If the signed agreement is silent on late fees, the number in the footer is a claim you would have to establish some other way, and in practice it produces an argument about the fee instead of payment of the principal. There is a second ceiling underneath the first one: interest rates are set by state law, the caps differ, and many states treat business debt on a different footing from consumer debt. The place to fix this is the payment clause before signature, not the invoice template afterwards.

The client's purchase order has its own terms and conditions attached. Did I agree to them?

Start with the master agreement rather than with the purchase order. The sentence you are looking for is the one declaring that pre-printed, additional or different terms on a PO are void and of no effect; where it exists, the attachment is noise and you can say so in one line. Where the agreement is silent, timing carries most of the weight. Quietly working on after the PO lands is what makes the argument hard later, and a short written reply in the same week, naming the agreement and declining the extra terms, is what keeps it easy. Two sentences now, or an hour with a lawyer admitted in the governing-law state afterwards, because the fallback rules on whose form governs are not the same in every state.

There is no signed contract at all, just my estimate and a go-ahead email. Do my invoice terms control?

This is the one situation where what you write on the invoice can genuinely matter, because there is no signed document sitting above it. What you have is an agreement assembled out of the estimate, the reply and the conduct of both sides, and the invoice is part of that record. It is a weaker foundation than a signed contract, and the terms you can realistically establish are the ones the client saw before the work started rather than the ones that first appeared on the bill. Some states now put the writing obligation on the client instead of on you, with consequences when the client cannot produce a contract.

Contract vs Invoice Payment Terms: Which One Wins

Q: The agreement says amendments must be signed. We have been changing things by email for a year. Where does that leave us?

Somewhere uncomfortable and state-specific. A clause requiring signed written amendments exists to stop exactly this, but a long run of behaviour that both sides accepted is the classic argument against reading it literally, and the drafting borrowed from the sales code carries that tension on its face: section 2-209(2) says a signed agreement excluding modification except by a signed writing cannot be otherwise modified, while 2-209(4) says an attempt at modification that fails those requirements can still operate as a waiver. That is goods law and your services contract is not governed by it, but the same argument gets made under common law, and how it lands depends on the state and on what the two of you actually did. If real money turns on a change nobody papered, take the thread to a lawyer before you invoice on the assumption it counted.

Section 7.2 of the master agreement: Payment shall be made net sixty (60) days following Client's receipt of an undisputed invoice. The invoice PDF, generated by the same accounting software you have used for four years: Terms: Net 15. 1.5% monthly interest applies to overdue balances.

Both documents are real. Both were sent by you. They describe two different deals, forty-five days apart, and one of them carries no weight at all. The uncomfortable part is that it is nearly always the one you control.

This page is only about the ranking question. Where the clock starts, once you know which terms apply, is a different problem, worked through in net 15, net 30 and due on receipt. I am not a lawyer and cannot tell you how it lands on your contract — only where to look, and what the public text said on the day I read it.

Where your invoice sits in the stack

For a typical engagement with a signed agreement in it, strongest paper first:

A signed amendment or change order executed after the agreement.
The signed master agreement itself.
The statement of work, where the agreement says the SOW governs its own subject matter — and the other way round where it does not.
Exhibits, attachments and rate cards incorporated by reference.
A purchase order, to the extent the agreement lets a PO carry terms at all.
Your invoice.
The proposal you sent before signature, which an entire agreement clause has probably erased.

Second from the bottom. That is not a rule of law but the ordinary effect of two clauses most client paper contains, which the section after next takes apart. One sentence to hold onto: an invoice is a demand for payment under an agreement, not a place to write one.

A bill is not a bargain

An invoice identifies who is owed, states the amount and what it covers, and triggers whatever internal process the client uses to release money. That last job is why invoice formatting matters enormously in practice and almost not at all in law.

What an invoice generally is not is an offer. An offer proposes terms for the other side to accept. By the time an invoice exists the parties have usually agreed already — the work is under way, the price was fixed somewhere else, and the document records a debt rather than proposing a deal. Paying it is performance, not acceptance of the small print in the footer. Which is why "Net 15" in the terms box of a template you downloaded in 2021 does not quietly rewrite section 7.2.

So changing your invoice template is not a contract strategy. Freelancers do it constantly, usually after a bad month: a late fee line, shorter terms, a bold warning above the total. None of it moves the due date when a signed agreement says otherwise.

The invoice is still evidence, though. A dated record of what you claimed and when, and a client who paid eleven invoices in the same format without ever objecting has done something a court can look at. Weak on its own. Not nothing.

Two clauses do almost all the work

Find both before you argue with anyone.

The entire agreement clause sits near the end, headed Entire Agreement, Integration or Merger. Standard shape: This Agreement constitutes the entire agreement between the parties with respect to its subject matter and supersedes all prior and contemporaneous proposals, understandings and agreements, whether written or oral. Its function is to draw a box around the signed papers. Anything you said in the pitch deck, the estimate or a Slack thread in March is presumed to sit outside that box.

The commercial code version of the idea is where the drafting came from. UCC 2-202 says terms in a writing intended as a final expression of the agreement may not be contradicted by evidence of a prior or contemporaneous oral agreement, though they may be explained or supplemented by course of dealing, usage of trade and consistent additional terms — unless the court finds the writing was meant as a complete and exclusive statement (read on 21 August 2026). Article 2 applies to transactions in goods (2-102), so it does not govern your retainer. The structure is the point: the tighter the integration language, the less room anything outside the document has to breathe.

The order of precedence clause is sometimes its own section, more often a subclause of Conflicts or Interpretation. It ranks the documents. To see one written plainly rather than in a client's house style, read the federal version. FAR 52.215-8 resolves any inconsistency in this order: the Schedule excluding the specifications, then representations and other instructions, then contract clauses, then other documents, exhibits and attachments, and last the specifications. The commercial-items version runs to nine rungs and ends the same way, at FAR 52.212-4(s). Both read from the eCFR text on 21 August 2026, when title 48 showed a latest issue date of 7 August 2026 and was marked current through 19 August 2026.

Notice what neither list contains. No invoices. A precedence clause ranks the documents that make up the contract, and an invoice is something produced under the contract, like a timesheet or a status report. It is unranked because it was never in the race.

What the contract says about changing the contract

The amendment clause is the one people skim. Typical text: No amendment or modification of this Agreement shall be effective unless in writing and signed by an authorised representative of each party. Some versions name who may sign, or require a specific form.

A written-amendment clause does not stop a client asking for changes informally. It stops those changes binding either side. When a producer emails "let's move you to net 30, easier for our system" and you say yes, you have an email, not an amendment — and when finance pays on day 58 they are performing the contract rather than breaching it. It cuts your way too: it is what you point at when a procurement manager announces net 75 across all suppliers next quarter. Announcements are not amendments. Neither is a banner in a portal.

Then there is the long-behaviour problem, where certainty runs out. A year of both sides acting as though a change was real is the standard argument against reading the clause literally, and the commercial code carries that tension openly: 2-209(2) says a signed agreement excluding modification except by a signed writing cannot be otherwise modified, while 2-209(4) says a failed attempt at modification can still operate as a waiver (read on 21 August 2026). Goods again. The same fight happens under state common law, and the outcome turns on the state and the facts. Here an hour of a lawyer's time is cheaper than being right in principle.

Terms that arrive after you already agreed

Purchase orders, portal click-throughs and the two-page Standard Terms for Suppliers PDF attached to an onboarding email share a feature: they appear after the deal is done and carry terms nobody negotiated. Payment terms, indemnities, audit rights, occasionally a different governing law.

For goods there is a famous rule. UCC 2-207 makes a definite and seasonable expression of acceptance operate as an acceptance even though it states terms additional to or different from those offered, unless acceptance is expressly conditional on assent to them; between merchants those terms join the contract unless the offer limits acceptance to its own terms, they materially alter it, or objection is given within a reasonable time (read on 21 August 2026).

Do not reach for it. Article 2 governs transactions in goods, and an edit, a design retainer or a copywriting engagement is a service. Services contracts stay under state common law, where the older rules live: in the traditional formulation an acceptance that changes terms is a counter-offer, and the party that sent the last form before performance began may find its terms are the ones that stuck — the "last shot" outcome described in this practitioner summary. Mixed jobs blur the line further, since a contract covering both goods and services is often classified by whichever predominates.

The practical move is not to win the analysis. It is to answer in writing the day the terms arrive, before you do more work: Confirming receipt of PO 44192. This engagement is governed by the MSA dated 4 March 2026; we are not agreeing to any additional or different terms in the purchase order or the supplier portal. Two sentences, sent once, and the question of who fired the last shot looks very different.

When your invoice terms are the only terms you have

There is a real case where the invoice carries weight: when nothing sits above it. No signed agreement, no SOW, just an estimate, a "yes, go ahead" and the work. The agreement is assembled from those pieces, and your paperwork is part of the record rather than a contradiction of it. Even here, what the client saw before starting outweighs anything that first surfaced on the bill.

A few jurisdictions have started shifting the writing obligation onto the client, which changes the picture again. Section 1412(3) of New York's General Business Law makes the hiring party retain the contract for six years and hand it over if the Attorney General asks, and not doing so raises a presumption that the freelance worker's version of the terms is the agreed one. Whether the thresholds and definitions reach your job is its own question, worked through in the two New York freelance laws.

Two lesser cases. Where the agreement is silent on a mechanical point — remittance details, invoice format, who to copy — the invoice fills a gap rather than contradicting a term, and gaps are easier to fill. And where a schedule says Payment terms as stated on Contractor's invoices, the invoice has been promoted by the contract itself. Rare. Worth asking for.

Everywhere else the invoice loses, which is the argument for spending the effort at signature, on the clauses mapped in the nine that decide whether you get paid.

Four lines to put in the next draft

Wording for a redline, adapted to the defined terms your client's paper uses. A lawyer in the governing-law state should look at it before you rely on any of it.

Precedence, with invoices named. In the event of a conflict, the following order of precedence applies: (a) any amendment executed by both parties; (b) this Agreement; (c) the applicable Statement of Work; (d) exhibits and attachments. Purchase orders, supplier portal terms and invoices are administrative documents and do not vary this Agreement.

Purchase orders and portals, closed off. No pre-printed, additional or different term contained in any purchase order, invoice, supplier portal registration, click-through agreement or similar document shall apply, and any such term is void and of no effect, whether or not acknowledged by either party.

Amendments, through a channel that exists. This Agreement may be amended only by a writing signed by both parties. For this purpose, an exchange of emails between the individuals named in Section [notices] that expressly identifies the section being amended shall constitute a writing. If your clients never open a signing tool, this beats the strict version — a clause nobody can comply with is one both sides quietly ignore, and that is how a year of unpapered changes builds up.

One line for the invoice itself, which does one narrow job: Issued under the Master Services Agreement dated [date], Section [x]. Payment terms per that Agreement. No new terms. It ties the bill to the document that holds the deadline, which is the quickest way to end a "we thought this was net 60" thread — and it removes the contradiction that opened this page.

None of this is legal advice, and the ranking of documents is decided by the governing law of your contract rather than by a rule that holds everywhere. If an invoice is caught between two of your own papers right now, gather two things before you call anyone: the entire agreement clause and the precedence clause, printed with their section numbers. That is the whole question in half a page.