Consultant Invoicing: Retainers, Milestones and Expenses

Mirror the statement of work

The fastest-approving consulting invoice is the one whose line items use the same words as the signed statement of work. If the SOW lists "Phase 2: current state assessment", that is the line item. Rewriting it as "analysis work" forces the approver to make a judgement call, and judgement calls take days.

Quote the SOW title, date and any contract or PO number in a reference line near the top of the invoice.

Retainer invoices

A retainer buys access or a capacity commitment, not a fixed output, so the line item should state the period rather than a deliverable: "Advisory retainer, March 2026, up to 20 hours". Where hours roll over or expire, say so in the notes.

Invoice retainers in advance on a fixed day of the month. Retainers billed in arrears drift and are the most common cause of a consultant working a full quarter before the first payment lands.

Milestone billing

Define each trigger as an event the client can verify. "Draft report delivered" is verifiable; "50 percent complete" is an argument.

Expenses and travel

Agree an expense policy in the SOW: which categories are recoverable, whether there is a cap, and whether receipts are required. Bill expenses on the same invoice as fees but in a clearly separated block, with tax handled according to your local rules.

Travel time is a fee item, not an expense. If you bill it, say so in the SOW rather than surprising the client on the invoice.

Surviving procurement

• Include the PO number exactly as issued, including leading zeros.
• Match the billing entity and address to the contract letter for letter.
• Do not exceed the PO value; raise a change order first.
• Send to the accounts payable address as well as your sponsor.
• Keep one invoice per PO where the client requires it.