Private Investigator Invoicing and Retainer Software (2026)
Investigative work is billed hourly against a prepaid retainer, plus mileage, database, and travel expenses, invoiced to law firms and individuals who need to see where the retainer went. ShowTheReceipts ($15/mo) takes the retainer up front, bills hours and expenses against it with the balance shown, references the case or matter on every invoice, and requests replenishment before the balance runs out.
How we picked
Entry-plan prices as published in 2026, taken from each vendor's own pricing page or a recent third-party summary. Judged on the solo investigator's workflow — retainers and replenishment, hourly and expense billing, case-referenced invoicing to firms, and confidentiality of the billing record itself — not on case management, evidence handling, or database search platforms, which are separate categories of product.
| Software | Starting price | Best for |
|---|---|---|
| ShowTheReceipts That's us | $15/mo | Solo PIs: retainers with running balance, hourly and expense billing, case-referenced invoices |
| Invoice2go | $9.99/mo+ | Basic mobile invoicing |
| Wave | Free / $16/mo | Free invoicing with light bookkeeping |
| FreshBooks | $21/mo+ | Time tracking with accounting attached |
| Clio | $49/mo+ | Law practices needing trust accounting and matter management |
The retainer is the engagement
Investigative work often turns up answers the client didn’t want to hear. Billing after the fact means asking someone to pay for news they’re unhappy about — probably the least collectible invoice in all of professional services.
A retainer, drawn down transparently, takes that problem off the table entirely.
1. ShowTheReceipts — $15/mo
Retainer taken up front, hours and expenses billed against it with the running balance shown, case reference on every invoice, replenishment requests at a stated threshold.
2. Invoice2go — $9.99/mo+
Basic mobile invoicing.
3. Wave — Free / $16/mo
Free invoices with light bookkeeping.
4. FreshBooks — $21/mo+
Time tracking with accounting attached.
5. Clio — $49/mo+
Built for law practices needing matter management and trust accounting.
The lines an investigative invoice needs
- Investigative hours — by activity type, with dates
- Surveillance hours — often at a separate rate, with a minimum
- Travel time — at full or reduced rate, stated in the engagement
- Mileage — per mile
- Database and records searches — at cost
- Court records, filings, and copies — at cost
- Report preparation — hourly or flat
- Testimony and deposition time — its own higher rate
- Retainer applied / balance remaining
Show the balance on every invoice
“How much of my retainer is left?” is basically the only recurring question in this business. Answer it on every invoice — opening balance, charges, closing balance — and you head off that whole category of dispute and make replenishment feel routine.
Keep the substance out of the billing record
Dates, hours, general activity, expenses. The invoice travels through offices, inboxes, and sometimes discovery, so it’s not the place for your findings.
Testimony is a separate product
Deposition and court time is scheduled, disruptive, and takes real skill. Price it above your field rate with a half-day minimum, and spell it out in the engagement letter so nobody’s surprised by it on the day.
Frequently asked questions
Why bill against a retainer instead of after the work?
Because investigative outcomes are frequently disappointing to the client, and collecting after a result nobody wanted is the hardest money in professional services. A retainer takes the payment question off the table before the work starts.
What should an investigative invoice show?
Dates, hours, a general activity description, expenses itemized, and the retainer balance before and after. Keep the substance of the investigation out of the billing document — it's a financial record, not a report.
How do I handle expenses?
Pass them through at cost on their own lines — mileage, database searches, court records, tolls, travel. Marking up expenses is discoverable and it damages credibility with the firms that send you repeat work.
When do I ask for replenishment?
Before the balance runs out, at a stated threshold written into the engagement. Stopping work mid-surveillance for a payment conversation is worse for the case and worse for the relationship than a scheduled top-up.