Sales Tax for Small Businesses: A Practical Guide to Nexus and Filing

Sales tax is one of those things that sneaks up on small-business owners. You start selling online, suddenly you have customers in several states, and before long you are wondering whether you owe tax to a government you have never interacted with. Here is how to think through sales tax collection without losing your mind.

What Sales Tax Nexus Actually Means

Nexus is just a connection between your business and a state that gives that state the right to require you to collect sales tax from your customers there. It used to be simple: if you had a physical store, office, warehouse, or employee in a state, you had nexus there. Then states started expanding the definition.

After the Supreme Court decision in South Dakota v. Wayfair in 2018, states began passing economic nexus laws. That means you can have sales tax obligations in a state even if you have zero physical presence there — purely based on how much you sell into the state or how many transactions you have with customers there.

How Economic Nexus Thresholds Work

Each state sets its own threshold. The most common pattern is a certain dollar amount of sales into the state within a calendar year, a certain number of separate transactions, or both. The specific numbers vary, so you need to check the rules in each state where you have customers.

The key point: you need to track where your customers are located and monitor your sales by state. If you cross a threshold in a given state, you may need to register for a sales tax permit there and begin collecting from customers in that state.

Collecting Sales Tax the Right Way

If you determine you have nexus in a state, here is the sequence to follow:

Register for a sales tax permit in that state before you start collecting. Collecting tax without being registered is illegal in most states.
Configure your sales channel to collect at the correct rate. Most e-commerce platforms can handle this, but you have to set them up. Sales tax rates vary not just by state but by county, city, and special district. Some states are origin-based, meaning you charge based on where you ship from. Most are destination-based, meaning you charge based on where the customer is located.
Track what you collect separately from your revenue. Sales tax you collect is not income — it is money you are holding on behalf of the state. Your bookkeeping should reflect this clearly. Set up a sales tax liability account and record collected tax there, not mixed into your revenue.

Remitting and Filing Returns

Once you are registered and collecting, you will need to file returns and remit the tax to each state. States assign filing frequencies based on your sales volume: monthly, quarterly, or annually. A return is usually due even if you collected zero tax during the period. This is called a zero-dollar filing, and skipping it can trigger penalties.

Keep a calendar of due dates by state. Late filings and late payments carry penalties and interest, and states tend to be aggressive about pursuing them.

Common Mistakes That Cost Money

Assuming you only owe tax in your home state. Economic nexus rules mean you could have obligations in many states where you have no physical presence at all.
Using the state rate instead of the combined local rate. Many cities and counties add their own tax on top of the state rate. If you only collect the state portion, you are shorting the local jurisdiction and may owe the difference out of pocket.
Collecting tax before registering. This is illegal in most states and creates a mess you have to unwind.
Treating collected sales tax as revenue. It is a liability, not income. Recording it correctly keeps your books clean and your profit numbers accurate.
Assuming your e-commerce platform handles everything automatically. Platforms have gotten better at calculating tax, but you still need to verify the settings, register in the right states yourself, and file the returns.

Keep Clean Records From the Start

Good sales tax hygiene comes down to clean records: knowing where your customers are, knowing where you have nexus, tracking what you collect by jurisdiction, and filing on time. This is one area where messy bookkeeping gets expensive fast, because the money involved is not yours to keep, and states will come looking for it.

If sorting through nexus thresholds, registrations, and liability accounts sounds like something you would rather hand off, that is exactly what TwoDayBooks does — keeping your books organized so sales tax and everything else stays under control.