State sales-tax automation after Wayfair
How the 2018 Supreme Court decision in South Dakota v. Wayfair rewrote the US state sales-tax regime — and how invoicing teams keep up with 45+ state DORs, economic nexus thresholds, and marketplace facilitator laws.
Pre-Wayfair: the Quill physical-presence rule
From 1992 to 2018, US state sales-tax obligations followed Quill Corp. v. North Dakota (504 U.S. 298, 1992): a state could only require a seller to collect its sales tax if the seller had a physical presence — an office, warehouse, employee, or inventory — within the state. The rule made remote selling tax-light: a Wisconsin direct-mail catalog house owed nothing to Wyoming as long as its operations stayed in Wisconsin. As e-commerce scaled in the 2000s and 2010s, states watched billions in sales-tax revenue evaporate into untaxed online transactions, and built up political pressure to revisit the rule.
The Wayfair decision (June 21, 2018)
The Supreme Court overruled Quill in South Dakota v. Wayfair, Inc. (138 S. Ct. 2080) on June 21, 2018. The 5-4 majority held that the physical-presence rule was 'unsound and incorrect' under modern Commerce Clause analysis. South Dakota's economic-nexus law — which required out-of-state sellers with more than $100,000 in sales or 200 transactions into South Dakota to collect — was upheld. Within months of the decision, every sales-tax-collecting state passed its own economic-nexus law modeled on the South Dakota template. Some states later raised the dollar threshold, dropped the transaction-count threshold, or both. The result is a patchwork that resembles the EU's pre-One-Stop-Shop fragmentation — but at the state level inside one country.
Economic nexus thresholds today
Most states use one of these patterns. (1) The South Dakota model: $100,000 in gross sales OR 200 transactions per calendar year. (2) Dollar-only: California ($500,000), Texas ($500,000), New York ($500,000 AND 100 transactions). (3) Sales-only with a transaction-floor: many smaller states. The threshold is per state, calculated annually, and resets each calendar year. Once you cross, you must register with that state's Department of Revenue, begin collecting on the next transaction (some states allow a 30-day grace), and file returns on the cadence the DOR assigns. Crossing the threshold also creates an enduring nexus until you formally close out the state.
Sourcing rules: destination vs origin
Two-thirds of US states use destination sourcing for sales tax: the rate is determined by where the goods are delivered to. The remaining states use origin sourcing for intrastate sales (the rate is determined by where the seller's office or warehouse is located) and destination sourcing for interstate sales. Texas is the most prominent origin state. The sourcing rule matters because it determines which jurisdiction's combined rate applies — a destination state with hundreds of local rates means your invoicing system must resolve the ship-to address all the way to county, city, and special district. For digital products and SaaS, sourcing rules vary further: customer-billing-address sourcing, customer-primary-use sourcing, or commercial-domicile sourcing.
Marketplace facilitator laws
Nearly every sales-tax state now has a marketplace facilitator law: if a platform like Amazon, Etsy, Walmart Marketplace, or eBay processes the transaction, the platform — not the seller — is responsible for collecting and remitting sales tax. This shifted billions in compliance burden away from individual sellers. The practical effect: if 100% of your sales go through such marketplaces, you typically have no economic-nexus collection obligation, but you must still understand the rules because the moment you sell directly (your own Shopify store, a B2B portal, a direct invoice), the threshold count includes those direct sales and you may suddenly cross.
The tax-engine architecture
Once you cross economic nexus in more than a handful of states, manual tracking is not realistic. The pragmatic architecture: your invoicing system owns the invoice and the customer relationship; a tax engine owns rate lookup, sourcing decisions, exemption-certificate management, and return filing. The leaders are Avalara AvaTax (the broadest coverage, mid-market and enterprise), TaxJar (Stripe-owned, SMB-friendly), Vertex (enterprise-only, deep customization), and Stripe Tax (built directly into Stripe Checkout). Your invoicing platform calls the engine's API at invoice time, gets back a sourced rate within milliseconds, applies it to the invoice, and the engine accumulates the transaction for month-end filing.
Audit defense and exemption certificates
State DOR audits focus on two things: are you collecting the right rate, and where you do not collect, can you prove the customer was exempt? Exemption certificates (resale certificates, manufacturing exemptions, government exemptions) must be on file before you treat a sale as exempt. A well-configured tax engine stores certificates digitally, ties each one to the customer, and flags certificates that have expired or are missing for sales claimed as exempt. During an audit, you export the audit trail: every rate decision, the geocode used, the product tax code applied, the exemption certificate referenced. A clean trail is the difference between a passable audit and a six-figure assessment.
The four leading US tax engines
Frequently asked questions
When does economic nexus reset each year?
Most states measure economic-nexus thresholds on a calendar-year basis: January 1 to December 31. Some use a rolling 12-month window (you cross when the trailing 12 months crosses the threshold). A few use the state's fiscal year. Your tax engine should track each state's specific measurement period — and alert you in real time as you approach the threshold in any state where you are not yet registered.
Do I owe back sales tax if I crossed nexus months ago and just realized it?
Yes. Once you cross economic nexus, your collection obligation begins (most states give a 30- to 60-day grace, but the principle is immediate liability). If you discover the crossing late, you typically owe the tax on all uncollected sales in the interim, plus penalties and interest. Voluntary Disclosure Agreements (VDAs) negotiated with the state DOR are the standard remediation — they cap back periods to 3-4 years and often waive penalties.
Is SaaS taxable in all 50 states?
No, and the patchwork is the worst part of US tax law. As of 2025, roughly half of US states tax SaaS as a taxable service or digital good; the other half treat it as nontaxable. Within taxable states, there are carve-outs for B2B vs B2C, on-premise vs cloud, and bundled vs standalone delivery. Your tax engine encodes each state's SaaS rule — keeping it current is the engine's job, not yours.
What if my customer claims they are tax-exempt?
Get the certificate before the next invoice. Standard exemptions: resale (the customer is buying for resale and will collect tax themselves), manufacturing (machinery and equipment used in production), government (federal, some states, some local), nonprofit (must register state-by-state). A signed exemption certificate has to be on file matching the customer and the transaction type. Your tax engine stores it, expires it on schedule, and prompts for renewal. Treating a sale as exempt without a certificate on file = assessment liability.
Can my invoicing software file state returns directly?
For one or two states with simple needs, yes — most modern invoicing platforms (4invoices included) offer in-built tax return filing for the smallest tier. For 10+ states, the architecture is: invoicing platform owns the invoice, dedicated tax engine (Avalara, TaxJar, Vertex, Stripe Tax) owns the returns. The engines specialize in keeping return formats current as state DORs update them — a job hard to do well as a side feature.