Sales tax for e-commerce comes down to one concept: nexus. Nexus means you have enough connection to a state that you’re required to collect and remit sales tax there. Before 2018, that meant physical presence. After the Supreme Court’s Wayfair decision, states can require collection based on economic activity alone.

Most states set economic nexus thresholds at $100,000 in sales or 200 transactions per year. A few use different numbers. When you cross that threshold in a state, you need to register, start collecting tax, and file returns. Miss this step and you’re accumulating liability every time you make a sale.

Here’s where it gets easier for some sellers. Marketplace facilitators like Amazon, eBay, Etsy, and Walmart are required to collect and remit sales tax on your behalf in most states. If you sell exclusively through these platforms, they handle the sales tax piece. You still need to track this for your records, but you’re not filing separate returns for marketplace sales.

Direct sales through your own website are different. Shopify, WooCommerce, BigCommerce, and similar platforms don’t collect sales tax automatically unless you configure them to. You’re responsible for determining where you have nexus, registering with those states, setting up correct tax rates in your platform, and filing returns.

The practical steps look like this. First, figure out where you have nexus by reviewing your sales by state. Many sellers are surprised to find they’ve crossed thresholds in states they never thought about. Second, register for a sales tax permit in each state where you have nexus. Don’t skip this step. Collecting without a permit can create bigger problems than not collecting at all. Third, configure your e-commerce platform to charge the right rates. Sales tax rates vary by state, county, and even city. Some products are exempt in certain states. Massachusetts exempts clothing under $175, for example. Fourth, file returns on schedule. Some states want monthly filings, others quarterly or annually, depending on your volume.

Getting sales tax compliance right from the start saves you from penalties and back taxes later. Tools like TaxJar and Avalara integrate with most e-commerce platforms and automate rate calculation, filing, and remittance. The subscription cost often pays for itself once you’re collecting in more than a handful of states.

The bigger risk isn’t getting audited. It’s accumulating years of uncollected tax liability because you didn’t realize you had nexus. States are getting better at identifying out-of-state sellers who should be collecting. Working with an Andover, MA bookkeeper who understands multi-state sales tax can help you get registered correctly and stay compliant as you grow.

If you’re selling online and haven’t looked at your sales tax obligations recently, start with a state-by-state sales report. That tells you where you stand today and what needs to happen next.