Agency Billing Guide (Retainers, Projects & Cash Flow)

The agency billing challenge

Agencies juggle three things at once: retainers that hit monthly, projects that run irregular, and pass-through expenses (media spend, freelancer costs) that inflate revenue on paper but not in the bank.

Get billing wrong and even a growing agency runs out of cash by Q3.

The 4 revenue streams every agency bills

Stream	Frequency	Best invoice model
Monthly retainer	Monthly	Auto-invoice on 1st
Project fees	Milestone-based	Deposit + milestones
Media / pass-through	Monthly (with report)	Client-net billing
Overages	Ad-hoc	Change orders, monthly true-up

Retainer billing done right

Structure: fixed monthly fee for a defined scope + hours cap.

Sample retainer invoice:

INVOICE #INV-2026-050
Date: Jun 1, 2026
Due: Jun 15, 2026 (Net 14)

Bill to: Acme Corp
Retainer: June 2026

Description                                        Amount
──────────────────────────────────────────────────────────
Marketing retainer — June 2026                    $8,000
  - Content strategy, SEO, monthly reporting
  - 40 hours included; overages billed at $175/hr
  - See engagement letter dated Mar 15, 2026

                                        TOTAL     $8,000

Key rules:

• Invoice on the 1st, due by the 15th — cash lands mid-month
• Include a specific hour cap (or "advisory" retainer with unlimited touchpoints)
• True up quarterly for overages or under-usage

Milestone project billing

For 3+ month projects:

Milestone	% of total
Kickoff / deposit	25-50%
Strategy / discovery complete	20-25%
Design / build midpoint	20-25%
Delivery / launch	15-25%
30 days post-launch (optional)	5-10%

This keeps cash flowing and creates natural checkpoints.

Pass-through expenses — the cash flow killer

If you spend $50,000 on Meta ads for a client and pass it through at cost, your P&L reads $50,000 revenue and $50,000 cost — but you paid the platform before the client paid you. That''s a 30-60 day cash gap.

Options:

1. Client-net billing — client pays media platforms directly (your favorite)
2. Prepaid media — client wires media budget upfront
3. Card float — you pay on a 30-day corporate card; client pays you Net 14; you clear the card in time
4. Markup (10-20%) to cover cash cost — becoming less accepted

Never pay $50k out of your operating cash on a client''s Net 60. That''s how agencies die profitable.

Change orders — the margin protector

Scope creep is the #1 cause of agency losses. Every out-of-scope request needs a change order:

CHANGE ORDER #CO-2026-014
Original SOW: Web Redesign (Apr 15)
Additional scope: Migration of 200 legacy blog posts
Additional fee: $2,400 (12 hours @ $200)
Additional timeline: +2 weeks

Client signs → you invoice → work begins. No sign, no work.

Agency invoicing checklist

• Retainer invoices sent by 1st of month
• Milestone invoices tied to deliverable acceptance
• Media pass-through documented with platform report
• Change orders signed before work
• Late payment reminders automated (see track overdue invoices)
• Aging report reviewed weekly

Cash flow rules of thumb

• Days sales outstanding (DSO) — target under 30 for retainers, under 45 for projects
• Cash reserve — 3 months of operating expenses minimum
• Media exposure — never more than 20% of monthly cash on pass-through

FAQ

Should agencies mark up freelancer costs? Yes — typically 20-40%. Transparent markup is standard; hidden markup damages trust. Some agencies bill freelancers at cost and charge a "management fee."

How do I handle a client who wants Net 60 or Net 90? Push back — offer a 2% discount for Net 14 (2/10 Net 30). If they insist on Net 60+, price it in (add ~3-5% cost of capital) or walk.

What if a retainer client goes quiet mid-month? Retainer stays billable — that''s the point of a retainer. Document it in the engagement letter: "Retainer bills regardless of usage. Overages billed separately."

Should I bill retainers in advance or arrears? Advance (1st of month, for that month) is standard and protects your cash. Arrears retainers create 60+ day cash gaps.

How do I structure invoicing for a 6-month project? Deposit (25-50%) + monthly progress invoices + final on delivery. See deposit invoice template.

Next steps

1. Audit your current retainer terms — hours cap, overage rate, true-up schedule.
2. Move media pass-through to client-net where possible.
3. Automate retainer invoices in Invoxa so they hit on the 1st every month without manual work.