How to invoice consulting retainers and advisory work
Consulting engagements are often high-value and long-running, and invoices are read by finance teams as well as the executives you advise. A clear, professional consulting invoice reinforces your expertise, aligns with the statement of work and keeps payments flowing on schedule.
For retainers, show the retainer as a single line with the period it covers in the notes. Bill any hours beyond the retainer separately with your hourly rate, and list project-based deliverables such as workshops, audits or reports as fixed-fee lines. This structure mirrors most consulting contracts and makes approval straightforward.
Pass-through expenses such as travel, accommodation and software should appear as separate lines, marked "at cost" if you don't add a markup. Keep receipts in case the client requests backup — many corporate clients require it.
Reference the contract or statement-of-work number and the client's purchase order on every invoice. Corporate accounts-payable departments frequently reject invoices that lack a PO, adding weeks to your payment cycle.
Set clear terms — Net 15 or Net 30 is typical — and state any late-payment interest permitted in your jurisdiction. Invoice on the same day each month for retainers so clients can plan, and keep every PDF for your records and tax filings.