Selling across state lines used to be simple from a tax point of view. You collected sales tax in the state where you had a store or an office, and that was that. Then the Supreme Court changed the rules in 2018, and now a business that ships orders into another state can owe sales tax there without ever setting foot in it. For owners running operations across several states, sales tax compliance has gone from a side task to a real part of running the business. This guide walks through what changed, what you owe, and how to keep up without losing your weekends to it.
Why Sales Tax Got Harder
The 2018 case South Dakota v. Wayfair set the rules everyone follows today. Before that decision, a state could only require you to collect sales tax if you had a physical presence there, like an office, a warehouse, or an employee on the ground. After Wayfair, states can require collection based on sales volume alone.
That means a business in Texas with no people, no buildings, and no inventory in Illinois can still owe Illinois sales tax once it crosses a certain threshold in sales to Illinois customers. Every state set its own threshold, and the rules keep shifting, which is why sales tax compliance now takes ongoing attention rather than a one-time setup.
Knowing Where You Have Nexus
Nexus is the word that decides where you owe tax. If you have nexus in a state, you have to register, collect, file, and remit. Two main types apply today.
Physical Nexus
This is the older standard and still in play. You have physical nexus in a state if you have an office, a store, a warehouse, an employee, a contractor, inventory, or even a server there. Attending a trade show in a state can create it. Storing goods in a fulfillment center counts. People often forget that selling through a marketplace that warehouses your inventory in many states can spread your physical nexus into all of them at once.
Economic Nexus
Economic nexus is the newer rule. It kicks in based on how much you sell into a state. The most common threshold sits around 100,000 dollars in sales or 200 transactions in a year, but plenty of states use different numbers, different time windows, and different ways of counting. A few states removed the transaction count and kept only the dollar figure. Others changed their numbers twice in the past few years. Watching where you are getting close to a threshold matters as much as watching where you have already crossed one.
Registering in Each State
Once you have nexus, you need a sales tax permit before you can legally collect tax. Collecting without registering is treated as worse than not collecting at all, since you are holding money that belongs to the state.
Registration is usually done online through the state revenue department. Each state has its own portal, its own forms, and its own filing schedule. Some assign your filing frequency based on your sales volume, monthly for higher volumes and quarterly or yearly for lower ones. Keep a master list of every state you are registered in, with the permit number, login details, and filing schedule, because tracking that across ten states from memory does not work for long.
Collecting the Right Rate
Sales tax rates are not one number per state. Most states layer state, county, city, and sometimes special district rates on top of each other. A buyer in one zip code might pay 6.5 percent, while a buyer two miles away pays 8.25 percent.
Most states use destination-based sourcing, which means you charge the rate at the buyer’s address rather than your own. A few origin-based states do the opposite. The rules for shipping charges also vary. Some states tax shipping, some do not, and some only tax it under certain conditions tied to how the order is itemized.
For any business handling more than a small number of orders, automated tax software pulling rates by address is the way most people manage this. Doing it by hand for a hundred orders a day is how mistakes pile up.
Filing & Remitting On Time
Filing a sales tax return is not the same as remitting the money. You usually do both at once, but the return itself shows what you collected and where, and the payment covers the amount due. States want both on time even when you collected nothing that period. A return showing zero is still required if you are registered.
Deadlines vary by state and by filing frequency. Late filings bring penalties that often start around 10 percent and grow from there. Several states also charge a separate late payment penalty, so a single missed deadline can hit you twice. A calendar with every due date, set up at the start of the year, is one of the easiest ways to keep sales tax compliance from slipping out of your hands.
Handling Exemptions & Resale Certificates
Not every sale is taxable. Sales to other businesses for resale, sales to nonprofits, and certain product categories are exempt in many states. When a customer claims an exemption, you need a valid certificate on file before you skip the tax. Without the certificate, an auditor will treat the sale as taxable, and you owe the tax yourself.
Certificates need to be current. Some states require renewal every few years, and certificates from one state are not always accepted in another. Setting up a system for collecting, verifying, and storing exemption certificates pays for itself the first time an audit notice lands.
Audits & Records
Sales tax audits happen, and they look at several years of activity at once. Most states keep records open to audit for three to four years, longer if they suspect you never registered when you should have.
Auditors want to see your sales records, your exemption certificates, your filed returns, and proof of payment. Records stored in one place and tied back to specific returns make the process smoother. Records scattered across emails, spreadsheets, and old software make it slower and more expensive every time someone has to dig.
Keep the documents that support every return for at least four years after filing, longer in states with longer lookback periods. Backups matter too, since digital files lost in a server crash are still your responsibility to produce when asked.
Staying Ahead of Changes
Sales tax rules keep moving. States add new tax categories. Rates change. Thresholds get updated. Marketplace facilitator rules shift which party is responsible for collecting on certain sales.
Setting aside time each quarter to review your nexus footprint catches changes before they cost you. Look at where your sales grew, check those states against their current thresholds, and confirm you are registered everywhere you need to be. For sales tax compliance, the cost of staying ahead is small. The cost of catching up after a state finds you first is not.
A Closing Thought
Selling into more states is good for the business. The tax work that comes with it is the price of that growth. Treat sales tax compliance the way you treat payroll, with a schedule, a system, and a person who owns it. The states are not going to call and remind you. They send a notice once it is already a problem, and at that point the fix costs more than the prevention ever would have.