The 7 Texas Sales Tax Mistakes Hiding in Your Clients' Contracts and Invoices

If your firm does the books, the invoicing, or the CFO work for Texas businesses, you see your clients' contracts and invoices before anyone else does — years before an auditor ever will. That's an advantage most firms never use, because the sales tax problems in those documents don't look like problems. They look like ordinary billing.

I spent my time at the Texas Comptroller building sales tax audits out of exactly these documents. Here are the seven patterns I saw create assessments over and over — what each one looks like in a file you already have open, why it happens, and the ten-minute check that catches it.

1. The contract says lump-sum, but the invoices bill separated

For construction and trade clients, the biggest single driver of assessments isn't a rate error — it's a mismatch. The contract says "total lump-sum price: $X" — but a document incorporated into the contract shows separately stated amounts: the invoicing software itemizes materials and labor, or a change order breaks the costs out. The Comptroller treats separately stated prices in any document that becomes part of the contract — including invoices and change orders — as creating a separated contract (34 TAC §3.291). The tax mechanics flip: who owes tax, on what base, and whether the client should have been collecting from customers.

The ten-minute check: pull one recent job. Put the signed contract next to the invoices actually sent. If one says lump-sum and the other itemizes, your client has been running a contract type they don't know they have.

2. "Remodel" on the invoice, "new construction" in reality — or the reverse

For work on nonresidential property, Texas draws a hard line: repair and remodeling of existing nonresidential property is a taxable service on the entire charge, while new construction labor is not taxable (34 TAC §3.357; Comptroller Pub 94-116). The words your client's estimator puts on the invoice — "remodel," "renovation," "repair," "build-out" — are the first thing an auditor reads, and they can drag a nontaxable job into the taxable column or flag a taxable job the client never collected on. And jobs that mix new construction with remodeling need each component accounted for separately — one invoice description can't cover both.

The check: scan invoice descriptions for nonresidential jobs. Does the language match what physically happened — first finish-out of new space versus modification of existing space? Sloppy descriptions cost real money in both directions.

3. Taxable and nontaxable services bundled into one line

When a single unseparated charge covers both taxable and nontaxable services — a taxable service bundled with exempt consulting, hardware with training, a maintenance agreement with monitoring — the Comptroller's rules presume that if more than 5% of the charge relates to taxable services, the entire charge is taxable unless the nontaxable portion is separately stated (see, e.g., 34 TAC §3.330(e) for data processing — the same presumption is written into the Comptroller's taxable-service rules across the board). Clients usually bundle because it reads cleaner on the invoice. The issue can be overcome in an audit if enough documentation exists to separate the charges after the fact — but that makes the audit take longer, creates unpacking work for your client, and leaves the result more exposed to auditor discretion. Every bundled line item is a small wager of the whole charge against the convenience of one line.

The check: look at your client's top five recurring invoice line items. For any line that mixes deliverables, ask: if the Comptroller taxed this entire line, what's the exposure across four years of that revenue stream?

4. SaaS, software, and "setup fees" treated as nontaxable services

Texas taxes data processing services — a category the Comptroller reads broadly enough to cover most SaaS, hosted software, and many "implementation," "onboarding," and "setup" fees that firms book as professional services (34 TAC §3.330). Twenty percent of a data processing charge is exempt (Tex. Tax Code §151.351), which trips clients in both directions: some tax nothing, some tax all of it. Either way, the invoice is wrong, and this is currently one of the Comptroller's favorite audit areas.

The check: if a client sells anything software-adjacent, find one customer invoice and ask what each line actually is: software access, SEO, web hosting, configuration labor, consulting, or training? If nobody can answer per line, neither can the invoice — and the auditor will answer it for them.

5. Zero-tax sales with no certificate in the file

Exempt and resale sales are only as good as the paper behind them (34 TAC §3.285, §3.287). In an audit, an exempt sale with no valid certificate on file becomes a taxable sale — and the auditor projects it across the sample. Clients lose certificates, accept expired ones, or take a customer's word for it. This is the single cheapest exposure to fix before a notice arrives, because gathering certificates under audit deadline pressure is a very different exercise. And a certificate only works if it's properly completed and supports a legitimate exemption — see the Comptroller's Form 01-339 (resale and exemption certificates) for what a valid one looks like.

The check: pull the client's exempt-sales report and match the top ten customers against the certificates on file. Every uncovered name — or improperly completed certificate — is exposure you can close this month for free.

6. No use tax accrual anywhere in the books

Out-of-state vendors, online purchases, equipment bought tax-free — when a seller doesn't charge Texas tax on a taxable purchase, your client owes use tax on it (34 TAC §3.346). Auditors go to purchases first, because it's the fastest schedule to build: pull the vendor list, find the out-of-state names, ask for invoices. If the chart of accounts has no use tax accrual line at all, that's not evidence of no liability — it's evidence nobody ever looked.

The check: search the GL for a use tax accrual account. If there isn't one, sort accounts payable by vendor state. Ten minutes tells you whether there's a problem worth quantifying.

7. The federal return and the sales tax reports tell different stories

Before an auditor reads a single invoice, they build a reconciliation: gross receipts reported on the federal income tax return, against total sales reported on the Texas sales tax returns for the same periods. It's the first schedule in the file, and an unexplained gap between the two becomes the spine of the entire audit. There are perfectly legitimate explanations — exempt revenue, out-of-state sales, timing differences — but your client needs to be the one holding the explanation, not the one hearing the question for the first time.

Two related patterns feed the same schedule. A sales tax payable account that keeps growing means tax is being collected from customers but not fully remitted — collected-but-unremitted tax is the finding auditors pursue hardest, because it's the state's money sitting in your client's account. And an accounting-method change in the last four years — especially a switch from cash to accrual — can throw the books and the sales tax reports out of alignment in ways that look exactly like underreporting until someone documents why.

The check: three quick passes. Reconcile gross receipts on the federal return to total sales on the sales tax reports and note the explanation for any gap. Scan the sales tax payable account — is the balance roughly one period's liability, or has it been quietly building? And confirm whether the accounting method changed during the last four years; if it did, write down the bridge now, not during an audit.

The one-page version

None of these require a specialist to spot. They're visible in documents your firm already touches every month. What they require is knowing the patterns — and having somewhere to send the ones that turn out to be real.

Sources & references

• 34 TAC §3.291 — Contractors
• Texas Comptroller — Real Property Repair and Remodeling (Pub 94-116)
• 34 TAC §3.357 — Nonresidential Real Property Repair, Remodeling & Restoration
• 34 TAC §3.330 — Data Processing Services
• 34 TAC §3.285 — Resale Certificates · §3.287 — Exemption Certificates
• 34 TAC §3.346 — Use Tax