--- title: "Retainer Billing for Agencies: Invoices & Overage | Fynex" description: "Retainer billing for agencies: recurring invoices, overage hours, set-up fees and milestones on one contract, VAT invoices paid by card or Faster Payments." url: "/uk/agencies/retainer-billing" --- Fynex is an AI-native finance operations platform — agentic finance — that automates and orchestrates end-to-end financial workflows for businesses. This page covers “Retainer Billing for Agencies: Invoices & Overage” as part of the Fynex finance operation. Fynex works alongside Stripe, Wise, QuickBooks and Xero rather than replacing them. This page is relevant when a business wants invoicing, payments, payouts, reconciliation and cash run as one automated operation rather than staffed across several tools. # Retainer billing that *invoices on the 1st*, every month Retainers, hours above them, set-up fees and milestones — billed from one contract per client and collected on a payment link. *Billing for agencies.* ## A retainer is recurring revenue billed like one-off work An agency retainer is a fixed monthly or quarterly fee for an agreed scope, often with hours or deliverables above it billed on top. In practice each month's invoice is raised by hand from a spreadsheet of what was delivered, the overage is added from a time tracker, and the set-up fee and the campaign milestone agreed in the SOW live in someone's memory. The retainer is predictable revenue; the billing of it isn't. UK agencies wait longest: late payment costs UK businesses almost £11bn a year and 86 hours each in chasing (London Economics, 2025), and in the FSB and GoCardless 2025 report 24% of small businesses were paid up to 60 days late. Statutory interest of 8% over base under the Late Payment of Commercial Debts Act only helps if the invoice went out on time with your terms on it. ## How retainer billing runs on Fynex One contract per client; the calendar raises the invoices. 1. **Recurring retainer invoices on the contract calendar** — Monthly, quarterly or any custom cadence, with notice periods and auto-renew — [recurring billing](/glossary/recurring-billing/) on the contract. Each period's retainer invoice is issued exactly once, on the day, numbered in a gapless series — whoever is on holiday. 2. **Hours and usage above the retainer** — Send hours or deliverables as usage events or a CSV from your time tracker. An included allowance covers the retainer; anything above it is rated at your overage rate and lands on the same client's invoice at period close. 3. **Set-up fees and milestones on the same agreement** — The onboarding fee bills once, the campaign milestone bills on sign-off, and the retainer keeps running — one contract, versioned, so a change of scope is an amendment rather than a new spreadsheet row. More on [deposits and stage payments](/blog/deposits-and-stage-payments/). 4. **Paid on a link, booked to your books** — Each invoice carries a hosted payment link for card, or the invoice number as the Faster Payments reference. Your payment terms and late-payment wording are on the invoice, and every payment is matched to it in Xero or QuickBooks. ## The same job, with the leaks closed Point tools solve part of it. Fynex runs the whole thing on one platform. | The problem | Other platforms (Xero / QuickBooks repeating invoices) | Fynex | |---|---|---| | Raise the monthly retainer invoice | A repeating invoice template, edited by hand each month. | Issued from the contract calendar, exactly once per period. | | Bill hours above the retainer | Copied from the time tracker onto the invoice. | Usage events or CSV rated against an included allowance. | | Bill set-up fees and milestones | Separate one-off invoices someone remembers to raise. | On the same versioned contract, each on its own trigger. | | Collect and reconcile | Bank transfer, matched by hand. | Payment link or transfer with the invoice number as reference, matched automatically. | ## Questions, answered ### What is retainer billing? Retainer billing invoices a client a fixed recurring fee — usually monthly or quarterly — for an agreed scope of work, sometimes with an allowance of hours or deliverables and overage billed above it. It gives an agency predictable revenue, provided the invoices go out on time and the overage is actually billed — the practical side is in [raise and chase the monthly retainers](/blog/retainer-billing-on-autopilot/). ### How do I bill hours over a retainer? Track the hours, compare them with the retainer's allowance, and bill the excess at an agreed overage rate on the next invoice. In Fynex, hours arrive as usage events or a CSV; the retainer's included allowance is applied first and the rest is rated automatically onto the client's period-close invoice. ### Can a retainer, a set-up fee and milestones share one contract? Yes. A Fynex contract can carry a recurring retainer, usage above an allowance, one-off fees and milestone payments, each billed on its own schedule. An amendment adds a new contract version, so every invoice traces back to the scope it was billed against. ### Can I charge late-payment interest on a UK retainer invoice? Under the Late Payment of Commercial Debts (Interest) Act 1998 a UK business can claim statutory interest of 8% over the Bank of England base rate on late B2B invoices, plus fixed compensation of £40, £70 or £100 depending on the debt. Fynex puts your payment terms and late-payment wording on every invoice and issues it on the due date, so the clock starts when it should. The escalation that follows: [late-payment letters that work](/blog/late-payment-letters-that-work/). ### Does retainer billing work with my accounting software? Yes. Invoices and payments sync to Xero and QuickBooks, each payment matched to the invoice that raised it, so the retainer revenue in your books is what was actually billed and collected. If contractors deliver part of the retainer, see [rebilling contractors](/agencies/rebilling-contractors/). Developers: the [subscriptions reference](https://docs.fynex.ai/billing-api/v1/docs/subscriptions). ## The rest of the agency billing layer Every guide below runs on the same contracts, invoices and payouts. Start with one, or run the whole stack. - **Rebilling contractors** — Rebill contractors and pass-through costs to clients with your margin separated on receipt — then pay each contractor. - **Subscriptions & Contracts** — The product page: renewals, proration, retries and hybrid contracts. ## Find what you're leaking, for free Send us your setup. We run a free diagnostic and show you what your payment stack costs you today. > One recent diagnostic found £2M in hidden fees across 10 processors for a proptech client. ## Your money, held under a licence, not a promise We can be your Merchant of Record, with your funds held by a licensed e-money institution. Most tools can't. - **Your funds, safeguarded** — Customer funds are held by Fondy Ltd, an FCA-authorised e-money institution (FRN 900705). - **PCI DSS Level 1** — The highest card-data security standard, on by default. - **Audit logs** — Every movement traceable and ready for review. - **Built-in KYC / KYB** — The switch that turns the business on. ## Retainers billed on the 1st. *Paid by the 5th* --- The AI finance layer for platforms and operators. It runs the money chain and keeps more of it in your business. One platform instead of a dozen. [Fynex on LinkedIn](https://www.linkedin.com/company/fynexai/) · [Fynex on X](https://x.com/FynexTeam) · [Fynex on GitHub](https://github.com/fynex-ai) · [Fynex on YouTube](https://www.youtube.com/@Fynex-AI) · [Fynex on Instagram](https://www.instagram.com/fynexteam/) © 2026 Fynex.ai Limited