Sales Tax Automation in a Custom ERP: Nexus, Rates and Filing for US Sellers

On 21 June 2018 the Supreme Court held in South Dakota v. Wayfair, Inc. that a seller does not need a physical presence in a state for that state to require it to collect tax. The South Dakota law at issue applied to sellers delivering more than $100,000 of goods or services into the state, or making 200 or more separate transactions there.

Those numbers are South Dakota's, not a national rule. Each state sets its own economic nexus test, and the tests differ in the amount, the measurement period and whether a transaction count is used at all. Two examples from state tax agencies:

• California requires a remote seller to register and collect use tax when combined sales for delivery in California exceed $500,000 in the preceding or current calendar year, according to the CDTFA. Sales through a marketplace count toward that total.
• Texas gives remote sellers a safe harbor under $500,000 of total Texas revenue in the preceding 12 calendar months, per the Texas Comptroller. The Comptroller says marketplace sales count toward the calculation even when the marketplace collects the tax.

Note the difference: one state uses calendar years, the other a trailing 12 months. A spreadsheet that applies one rule to all states will be wrong somewhere. Check each state's revenue department site as of October 2026 before you set rules.

The ERP already holds the data needed to see where you are heading: orders by ship-to state. A nexus monitor turns that into an alert before you cross a threshold, not after.

Design it as a rules table, one row per state, with fields for the dollar threshold, any transaction threshold, the measurement window (calendar year or trailing months), whether marketplace sales count, and whether nontaxable or resale sales count. California's guidance, for instance, says the sales total can include nontaxable sales such as sales for resale. The monitor then computes each state's running total on the right basis and flags states at 70%, 90% and 100% of the limit.

Two cautions. First, physical nexus still exists: employees, inventory in a warehouse, or a fulfillment center in a state can create a collection duty regardless of sales volume. Record where your people and stock sit. Second, marketplace-heavy sellers need a clear data feed: your own ERP will not see sales that a marketplace facilitator collects tax on unless you import them, yet those sales can still count toward your threshold.

Rates vary by state, county, city and special district, and the rate for an order depends on the ship-to address. Do not maintain a rate table by hand. The Streamlined Sales Tax governing board says participating states maintain free rate and boundary databases that map a ZIP code to taxing jurisdictions and their rates, and that a business relying on the state's files is held harmless for over- or under-collection. Certified service providers (CSPs) offer the same job as a paid service with returns support. Commercial tax engines also exist, and the Streamlined site lists certified providers.

Your ERP should therefore call a tax service at order time, not store rates locally. The call sends the ship-from address, ship-to address, product tax category, customer exemption status and amount; it gets back the jurisdictions and tax per line. Cache results briefly, but always store the response on the invoice.

What you sell matters as much as where. States tax goods, software, digital products and services differently, and clothing, groceries and medical items often have special treatment. Give each item in your catalog a tax category that maps to the tax service's codes, and make it a required field when someone creates a product. Untagged items are the most common source of wrong tax.

For business customers, handle exemption certificates. Resale and other exemptions mean you do not charge tax, but you need the certificate on file. Build a certificate store linked to the customer record with state, type, issue date and expiry, and make the order screen apply the exemption only when a valid certificate exists. Auditors ask for these, and a missing certificate can turn an untaxed sale into your liability.

An invoice should keep not just the tax amount but the inputs behind it: the address used, the jurisdiction codes, the rate, the product category, the exemption reason and the date of the rate file. Years later, you will be asked why you charged a certain amount on one order. If the ERP stores the evidence, you can answer in minutes. Texas, for example, requires sellers to keep marketplace sale records for at least four years, according to the Comptroller's remote seller guidance, and other states set their own retention rules.

Handle changes properly too. A refund or return should reverse the tax with the same jurisdiction split as the original sale, and a credit note should reference the original invoice.

The ERP should produce a per-state, per-period report of gross sales, exempt sales, taxable sales and tax collected, split by jurisdiction where the state requires it. From there you have a choice:

• Export the report and have your accountant or a filing service submit the return.
• Use a tax provider's filing service via API, with the ERP sending a locked period of transactions.
• File directly through each state's portal, which works for a few states but scales poorly.

Filing frequency depends on the state and your volume, so store each registration's filing frequency and due dates, and surface an upcoming-deadline view. Reconcile collected tax in the ERP against the amount in your general ledger liability account every period. Differences usually mean a product miscategorized or an order edited after invoicing.

Writing your own tax rules engine is a poor use of money; rates and taxability change constantly and tax providers handle that. What a custom ERP adds is workflow: nexus alerts that reflect your own channels, exemption certificates tied to your customers, the audit trail, and the connection to your inventory and accounting. See our US ERP development page and the main ERP development service page for how we scope such systems. For an example of tax logic handled in software, our AccVora case study covers GST billing for Indian businesses; it was built for India's tax system, not for US sales tax, but it shows how invoice-level tax rules are structured.

Do I owe sales tax if I only sell on Amazon or Etsy?

Marketplace facilitator laws mean the marketplace usually collects tax on your behalf in most states, but your sales can still count toward your own threshold, as the Texas Comptroller states. Direct-website sales are your responsibility.

Is there one national economic nexus threshold?

No. Wayfair upheld South Dakota's approach, and each state sets its own test. Check each state revenue department's current guidance.

Can the ERP file my returns automatically?

It can prepare the report and, through a tax provider's API, submit it. Many sellers keep a human review step before filing.

How long does a build like this take?

A nexus monitor and tax-engine integration can often be scoped as one phase of an ERP project, with working software on staging every two weeks. We confirm the schedule in the written scope after discovery.

Should I register in a state before I hit the threshold?

That is a decision for your accountant. Registering earlier can avoid back-tax risk, but it creates filing duties.

If you sell across state lines and want to know what your ERP should track, start with a discovery call on our contact page. We will map your channels and states, and send a written scope with a fixed quote.