Payment Terms in Contractor Contracts: Net-30, Milestones, Retainer Compared

A US founder hires a developer overseas at a USD 6,000 monthly rate. The contract says “Net-30 from invoice date.” Three months in the founder has not paid for month two and is now overdue on month three. The contractor stops work. The founder argues the invoice was unclear. The contractor argues the invoice was clear and the deemed-accepted window had passed. The contract has no deemed-accepted clause. The dispute drags for six weeks. The product launch slips. Both sides lose.

This is what payment terms look like when nobody reads them at signature. This guide walks through the four common contractor payment structures (Net-30, Net-60, milestones, retainers), where each one fits, late fee enforceability under US state law, invoice acceptance procedures, and how to draft partial-payment treatment that holds up under scrutiny.

The four payment structures

Net-30 (the US default)

Net-30 means the invoice is due 30 days after issuance. It is the de facto US business standard, comes from purchase-order accounting practice, and is what most accounting platforms default to.

When it fits:

• Hourly or time-and-materials engagements with regular monthly invoicing
• Established contractor relationships where the client has a reliable payment history
• Engagements small enough that a one-month float does not strain the contractor

Failure modes:

• Contractors with thin runways cannot float a month. They will quote higher rates to absorb the risk.
• Without a deemed-accepted clause, “Net-30” silently becomes “Net-when-the-AP-team-gets-around-to-it.”
• Approval workflows that require sign-off from three people kill Net-30 in practice. The contract says 30, the reality is 50.

Net-60 and Net-90 (the leverage move)

Net-60 and Net-90 are common in enterprise procurement and in some industries (consumer products, retail, ad agencies). They push working-capital cost onto the supplier.

Cost: A Net-60 term effectively borrows 30 extra days of money from the contractor. At a 10 percent annual cost of capital, that is roughly 0.8 percent of contract value per Net-30 stretch. Sophisticated contractors price this in. Less sophisticated ones absorb it as margin compression. Either way the client pays, just not on the invoice.

When it fits:

• Large established contractors with deep balance sheets
• Engagements where the contractor uses the client logo for credibility and trades that for slower payment
• Industries where Net-60 is genuinely standard

Failure modes:

• Net-60 with small contractors creates resentment and quality drops
• It signals that the client does not respect cashflow, which can affect future contractor recruitment

Milestone-based payments

Milestone payments tie cash to acceptance of specific deliverables. A typical structure is a 20-30-30-20 split across kickoff, mid-build, code-complete, and final acceptance.

When it fits:

• Fixed-fee project work with clear deliverables
• Engagements where scope is known at the outset
• High-trust relationships where both sides agree on what “done” means

Failure modes:

• Without testable acceptance criteria, milestone disputes turn into stalemates
• Milestone gating without a deemed-accepted window lets the client stall on the last 20 percent indefinitely
• Mid-build milestones often get paid before any verifiable proof of progress, creating moral hazard on the contractor side

A milestone schedule is only as strong as its acceptance criteria. Testable conditions (the API returns 200 for the documented endpoints, the design files are delivered in Figma, the report covers the five required sections) work. Adjectives (“the work is satisfactory”) do not.

Retainers

A retainer is a recurring fixed payment for ongoing availability, regardless of hours worked or deliverables produced. Common in legal, fractional executive, advisory, and ongoing-operations engagements.

Structure variants:

• Pure capacity retainer: Pays for X hours of availability per month, used or unused. Unused hours do not roll over.
• Minimum retainer: Pays for X hours, with overflow billed at a separate rate.
• Banked retainer: Pays for X hours that accumulate if unused, with a cap.

When it fits:

• Fractional CTO, CFO, general counsel, or advisor roles
• Ongoing operations support (managed services, on-call engineering)
• Senior advisors where the value is being available, not being billable

Failure modes:

• A retainer that looks like a salary (paid monthly, no specific deliverables, the contractor reports to a manager) creates serious misclassification risk under the IRS common law test, the FLSA economic reality test, and California’s ABC test under Labor Code 2775. See contract renewal strategies for long-term contractors.
• Retainers without scope language drift into open-ended employment substitutes.

Late fees and interest: what is actually enforceable

The temptation is to write a 5 percent monthly late fee. The reality is that courts strike penalty clauses.

The reasonableness rule

Under UCC 2-718(1), liquidated damages “may be liquidated in the agreement but only at an amount which is reasonable in the light of the anticipated or actual harm caused by the breach” (https://www.law.cornell.edu/ucc/2/2-718). “A term fixing unreasonably large liquidated damages is void as a penalty.”

California Civil Code Section 1671(b) applies the same rule to commercial contracts: a liquidated damages provision is valid unless the challenging party proves it was “unreasonable under the circumstances existing at the time the contract was made” (https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CIV&sectionNum=1671.).

What rates hold up

Enforceable late fees generally fall in three patterns:

1. Reference rate plus margin: Prime rate plus 2 to 4 percent annualized, applied to the overdue balance. Easy to justify because it tracks the cost of money.
2. Fixed monthly percentage in the 1 to 1.5 percent range: Roughly 12 to 18 percent annualized. Within most state usury limits and roughly matches commercial lending rates.
3. Statutory rates where they apply: US federal procurement uses the Treasury rate set under 31 USC 3902 (https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title31-section3902). For 2026 the published rate is 4.125 percent annualized. State construction prompt-payment laws set their own rates (often 1 to 1.5 percent per month).

State variations to know

• Texas: The Texas Prompt Payment Act (Chapter 28 of the Texas Property Code) applies to construction. For private construction, interest on undisputed late payments accrues at 1.5 percent per month (https://statutes.capitol.texas.gov/Docs/PR/htm/PR.28.htm). Outside construction, Texas defaults to UCC reasonableness.
• California: Civil Code 1671(b) governs commercial liquidated damages. Construction prompt-payment is in Business and Professions Code 7108.5 and Public Contract Code 10262.
• New York: General Obligations Law and CPLR 5004 set 9 percent statutory interest as the default for breach of contract claims (https://www.nysenate.gov/legislation/laws/GOB/5-901).
• Federal contractors: The Prompt Payment Act (31 USC 3902) controls, with the Treasury-published rate.

Drafting tip

Write the late fee as “the lesser of 1.5 percent per month or the maximum rate permitted by law.” This automatically caps you at the state usury limit and survives a penalty challenge in most jurisdictions.

Invoice acceptance procedures

The payment-term clause is half the picture. The other half is how invoices get accepted.

Required elements

1. Submission method: Email to a defined address, portal upload, or both. Specify.
2. Required contents: Invoice number, period covered, line items, hours (if applicable), reference to PO or SOW, payment instructions.
3. Review window: 10 to 15 business days is standard.
4. Dispute mechanics: A written objection within the window with specific line items disputed. Undisputed amounts must be paid on schedule.
5. Deemed-accepted clause: If no written objection within the window, the invoice is deemed accepted and due.
6. Cure period for disputes: 15 to 30 days to resolve, after which the contractor can suspend work or terminate.

Sample clause

Contractor shall submit invoices monthly to ap@client.com referencing the applicable SOW. Client shall review each invoice within 10 business days of receipt. If Client does not provide a written objection itemizing specific disputed amounts within the review window, the invoice is deemed accepted in full and due on the original payment date. Client shall pay all undisputed amounts on schedule notwithstanding any dispute on other line items.

This clause does three things: it sets a definite review window, it shifts the risk of silence to the client, and it isolates disputes so that the rest of the invoice still flows.

Partial payment and accord and satisfaction

A client paying USD 8,000 on a USD 10,000 invoice can create a problem if the payment is treated as “full and final settlement.” This is the doctrine of accord and satisfaction.

Under UCC 1-308 a party may “perform or promise performance or assent to performance in a manner demanded or offered by the other party without thereby prejudicing the rights reserved.” Spell this out in the contract:

Any partial payment by Client shall not constitute acceptance of the invoiced amount in full or waiver of Contractor’s right to collect the balance. No payment shall operate as an accord and satisfaction unless the parties so agree in a separate writing.

This blocks the “we paid you, you cashed the check, take it or leave it” argument.

Choosing between structures

How Omnivoo handles payment terms

Omnivoo’s Contract Management product ships templates for each of the four structures with built-in deemed-accepted clauses, dispute mechanics, and partial-payment language. Late fee defaults are set to “lesser of 1.5 percent per month or maximum permitted by law” so they stay enforceable across US states. The platform tracks invoice acceptance windows automatically and flags overdue invoices once the deemed-accepted period closes.

For broader contract structuring, see drafting a SOW for US companies hiring global contractors and contract change orders. For misclassification-aware retainer design, see the Contract Management product page.

Drafting checklist

• Is the payment term explicit (Net-30, Net-60, milestone schedule, monthly retainer)
• Does the invoice clause specify submission method, required contents, and review window
• Is there a deemed-accepted clause with a defined window
• Are dispute mechanics defined (written objection, itemized, cure period)
• Is the late fee capped at “lesser of X or maximum permitted by law”
• Is there an accord-and-satisfaction blocker for partial payments
• For milestones: are acceptance criteria testable, not adjectival
• For retainers: does the structure avoid looking like a salary

If you remember three things

1. Net-30 is a convention, not a default. Write down what you actually mean, including review windows and deemed-accepted timing.
2. Late fees must be reasonable. A 5 percent monthly rate is almost certainly a penalty and will be struck. Use 1 to 1.5 percent per month tied to a published reference rate.
3. Acceptance procedures are where payment terms succeed or fail. Without a deemed-accepted clause, Net-30 silently becomes Net-whenever.

Good payment terms are not the most exciting part of a contract, but they are where most contractor disputes actually start. Get them right at signature and the rest of the relationship runs smoother.