The most common sales tax mistakes are collecting only where you have an office, charging one flat rate everywhere, ignoring use tax on your own purchases, relying on taxability rules that have since changed, and accepting exempt sales without a valid certificate. Each one leaves the business paying tax out of its own pocket once an auditor finds it, usually with penalties and interest on top.
This guide explains how those common sales tax mistakes happen, what the 2026 rules look like in a few states that changed recently, and how to build a monthly review that catches errors before they compound. A worked example shows how a modest gap turns into a real bill.
Sales tax is a pass-through tax. The business collects it from customers and sends it to the state, so in normal operations it never touches profit. That changes the moment something goes wrong. If you failed to charge tax on a sale that was taxable, the state still expects the money, and the customer is long gone. The tax then comes out of your margin.
Late filing adds a second layer. In Texas, for example, the Comptroller's sales tax page states that "A $50 penalty is assessed on each report filed after the due date," that tax paid 1 to 30 days late carries a 5 percent penalty, and tax paid more than 30 days late carries a 10 percent penalty. Interest is charged separately, and the Comptroller notes that "Past-due taxes are charged interest beginning 61 days after the due date." Other states use different percentages, but a flat late fee plus a percentage penalty plus interest is a common pattern.
Take a business that sells into Texas, where the state rate is 6.25 percent and local jurisdictions can add up to 2 percent "for a maximum combined rate of 8.25 percent," according to the Comptroller. The figures below assume the full 8.25 percent applies and ignore interest, which varies by period.
| Scenario | Base amount | Calculation | Cost to the business |
|---|---|---|---|
| Taxable sales billed with no tax collected | $200,000 | $200,000 x 8.25% | $16,500 of tax paid from your own funds |
| Out-of-state equipment purchase with no tax charged | $12,000 | $12,000 x 8.25% | $990 of use tax owed |
| Return with $10,000 of tax filed and paid 20 days late | $10,000 | ($10,000 x 5%) + $50 | $550 in penalties |
| Return with $10,000 of tax filed and paid 45 days late | $10,000 | ($10,000 x 10%) + $50 | $1,050 in penalties |
The first row is the one that hurts. A business that never collected tax cannot go back to most customers for it, so an audit assessment on uncollected sales becomes a direct expense. That is why most of the fixes below focus on getting the setup right before invoices go out.
Many businesses still register only in their home state. That view has been outdated since 2018, when the Supreme Court decided South Dakota v. Wayfair. The South Dakota law at issue covered sellers that "on an annual basis, deliver more than $100,000 of goods or services into the State" or "engage in 200 or more separate transactions for the delivery of goods or services into the State," and the Court held that "the physical presence rule of Quill is unsound and incorrect," per the Wayfair opinion syllabus. Every state with a general sales tax now has an economic nexus rule of some kind.
The second part of this mistake is assuming the thresholds stay fixed. States keep adjusting them. Illinois is a current example: its Department of Revenue explains in Bulletin FY 2026-12 that "On or after January 1, 2026, the only threshold used to determine if a remote retailer or marketplace facilitator" has to collect is $100,000 in cumulative gross receipts. The separate 200-transaction test no longer applies there.
How to fix it: run a sales-by-state report at least quarterly and compare it with each state's current threshold and measurement period. Our guide to economic nexus for growing businesses covers how to set up that tracking. If you find a state where you crossed a threshold in a prior year and never registered, a sales tax voluntary disclosure agreement is often the cleanest way to come forward.
Another frequent shortcut is charging a single state rate, or the rate at your headquarters, on every order. In many states the total rate depends on the combination of state, county, city and special district rates at the point of delivery. Texas alone allows local add-ons from cities, counties, special purpose districts and transit authorities on top of the state rate.
The rule that decides which location counts is called sourcing. Most states use destination sourcing for shipped goods, while a handful use origin sourcing for some in-state sales. Our article on destination-based and origin-based sales tax sourcing explains how to tell which rule applies to an order.
How to fix it: use address-level rate lookup through your ecommerce platform or a dedicated tax engine, and test a few addresses each quarter against the state's own rate lookup tool. If you are comparing engines, our review of sales tax automation software lists what each one handles. Software only helps when product tax codes and ship-from addresses are configured correctly, so build that check into onboarding for any new product line.
Use tax is the counterpart to sales tax, and it applies when your business buys taxable items without paying sales tax at checkout. Common examples include equipment bought online from out-of-state vendors, software licenses and office supplies from sellers that are not registered in your state.
The Texas Comptroller's use tax page puts it plainly: "A Texas purchaser owes state and local use tax if they buy taxable goods and services" and when the seller does not collect it, "the purchaser must pay the use tax directly to our office unless an exemption applies." Businesses that already hold a Texas permit report these amounts on their regular sales and use tax return as taxable purchases.
Auditors look for this early because it is easy to test. They pull your fixed asset additions and large expense invoices and check whether tax appears on each one.
How to fix it: add a monthly accounts payable review of invoices from out-of-state vendors. Flag any taxable purchase with no sales tax line, and accrue use tax at your local rate on the next return.
Taxability rules change more often than rates do, and the changes tend to hit services and digital products. A business that set up its tax codes five years ago may now be undercharging without knowing it.
Washington State is a clear 2025 example. The Washington Department of Revenue states that "As of Oct. 1, 2025, a new law (ESSB 5814) requires that certain services now be taxed when sold." The list includes advertising services, information technology services, custom website development, live presentations, investigation and security services, temporary staffing, and custom software and customization. A consultancy or agency that billed Washington clients tax-free in mid-2025 may need to charge tax on the same work today. Our Washington sales tax guide covers the change in more detail.
Software is another moving target. States treat downloaded software, SaaS and digital subscriptions differently, and several have revised their positions in recent years. Our guide to SaaS sales tax explains how to map a subscription product state by state.
How to fix it: schedule a taxability review whenever you add a product type, start billing a new kind of service or enter a new state, and at least once a year otherwise. Document the source for each decision so you can show an auditor why you treated an item the way you did.
Exempt sales without proper paperwork are a classic audit finding. Treating "the customer said they are exempt" as enough is risky, because most states presume a sale is taxable unless the seller holds a valid resale or exemption certificate. Texas spells this out in its exemption certificate rule (34 TAC 3.287): "All gross receipts of a seller are presumed subject to sales or use tax" unless "a valid and properly completed resale or exemption certificate is accepted by the seller." The same rule requires the certificate to be accepted at or before the time of the transaction and kept for at least four years from the date of the sale. When the certificate is missing or incomplete, an auditor will generally treat the sale as taxable and assess the seller.
How to fix it: collect the certificate before the first exempt invoice, check that the form matches the state where the sale is delivered, and store certificates where your billing team can see them. States that belong to the Streamlined Sales Tax agreement accept a uniform exemption certificate, which our Streamlined Sales Tax explainer describes. Set a reminder to renew certificates in states that require periodic updates.
Two reconciliation errors show up often in ecommerce books. The first is refunding the product price to a customer while still reporting the original tax as collected, or the reverse, adjusting tax on the return without having refunded it. The second involves marketplace sales. In states with marketplace facilitator laws, the marketplace collects and remits tax on sales it facilitates, so sellers who also report that tax on their own returns end up paying twice. State rules differ on whether marketplace sales still count toward a seller's own nexus threshold, so check each state separately.
How to fix it: reconcile the tax liability account to filed returns every month, split marketplace sales from direct sales in your reports and match refunds to their original tax treatment. Our article on ecommerce returns, chargebacks and sales tax reconciliation walks through the journal entries.
Sales tax returns are due monthly, quarterly or annually depending on the state and your volume, so errors compound between filings. A business that only reviews its sales tax position at year-end gives small mistakes up to twelve months to grow, and fixing them later is harder because the people involved may not remember why certain choices were made.
A short monthly or quarterly routine works better. It should cover four checks: rates charged against current rates, returns filed against the tax liability account, new exempt customers against certificates on file, and sales by state against nexus thresholds. Our sales tax compliance checklist for small businesses turns these checks into a recurring list.
Most of these errors share a root cause: the setup made sense when the business was smaller, and nobody revisited it after growth or a change in law. The table below links each mistake to the control that prevents it.
| Mistake | Control | Suggested frequency |
|---|---|---|
| Missed economic nexus | Sales-by-state report compared with thresholds | Quarterly |
| Flat rate on every order | Address-level rate lookup and spot checks | Quarterly spot test |
| Unpaid use tax | Review of out-of-state vendor invoices | Monthly |
| Outdated taxability | Product and service taxability review | Annually and at each launch |
| Missing exemption certificates | Certificate collected before first exempt invoice | At onboarding |
| Return and marketplace errors | Liability account reconciliation | Monthly |
If a state does contact you, our guide on how to handle a sales tax audit explains what to prepare and how to respond.
What is the most common sales tax mistake for small businesses? Missing economic nexus in states where the business has no office is one of the most frequent and most expensive errors. Because the business never collected tax in that state, any assessment usually comes out of its own funds.
Can I recover sales tax I forgot to charge a customer? You can ask the customer to pay it, but many will refuse, especially on older invoices. In practice, the business usually absorbs the tax, which is why correct setup before invoicing matters.
Do I owe use tax on software bought from an out-of-state vendor? It depends on whether your state taxes that type of software. If it does and the vendor did not charge tax, the purchaser generally owes use tax directly to the state.
What should I do if I discover past sales tax errors? Quantify the exposure by state and period first. For states where you never registered, a voluntary disclosure agreement is usually worth considering. Under the Multistate Tax Commission's voluntary disclosure program, participating states waive tax for periods before the lookback period and waive penalties for the lookback period, though interest is generally still due. Each state sets its own terms. For states where you are registered, amended returns are the usual route.
Madras Accountancy supports businesses and CPA firms with sales tax work, including nexus reviews, return preparation, use tax accruals and liability reconciliations, alongside bookkeeping and tax preparation. To talk through your sales tax setup, contact our team.
The section 4960 excise tax is 21% of nonprofit pay over $1M. From 2026 it covers all employees. See the medical carve-outs, Form 4720 and a worked example.
The paid family and medical leave tax credit is permanent from 2026. See the 12.5% to 25% rates, the premium option, the 6-month rule and how Form 8994 works.
Opportunity zones become permanent in 2027 under the One Big Beautiful Bill. See the new deferral, rural bonus, and the 2026 handoff.
