How far back do you owe sales tax after discovering you crossed a state threshold two years ago?

Size the Exposure Before You Do Anything Else

The first instinct is to register immediately and start collecting. Resist it for a week. Registering sets a start date and often asks when you first had nexus, and answering that question badly can turn a manageable cleanup into an open ended one. Instead, export every order for the last three or four years with ship to state, date, order subtotal, shipping, and channel. Sum by state and month. You are looking for the month each state's sales or transaction threshold was actually crossed, because your obligation starts from that point plus whatever grace the state allows, not from the day you noticed. Related: What Economic Nexus Means and Why It Matters

Then shrink the number honestly. Sales made through a marketplace that collected on your behalf carry no tax liability for you, although some states still count them toward the threshold. Sales to customers who were genuinely exempt can be backed out if you can obtain certificates now, and many wholesale buyers will provide one retroactively when asked. Products that are not taxable in that state come out too, which matters a lot for categories like groceries, clothing, or certain digital goods. What is left is your real exposure, and in our experience the first honest pass usually lands well below the panicked estimate. Related: Sales Tax and Marketplaces: Who Collects

Keep reading: US Sales Tax Basics for Online Sellers, What Economic Nexus Means and Why It Matters, Origin vs Destination Based Sales Tax, Explained. See how SalesTaxly helps you us sales tax rates and lookup for online stores.

Plain Registration or a Voluntary Disclosure Agreement

For a registered filer who simply filed late, states generally have a limited lookback, commonly three or four years. For a seller who never registered at all, most states can look back as far as they like, because the clock on a return that was never filed never started. That asymmetry is the entire reason voluntary disclosure programs exist. A voluntary disclosure agreement, usually called a VDA, is a deal: you come forward, the state caps the lookback at a fixed period that is typically three to four years, waives penalties, and you pay the tax plus interest and start filing going forward.

Two conditions matter. First, a VDA is normally only available before the state contacts you, so a nexus questionnaire or an audit notice in your inbox usually closes the door. Second, many states run the process anonymously through a representative until terms are agreed, which is why sellers with real exposure use a state and local tax advisor rather than filing the forms themselves. If the exposure in a state is small, say a few hundred dollars of tax, the paperwork and professional fees can exceed the liability, and simply registering and filing the back periods is often the cheaper honest answer. Run that comparison state by state, not once for the whole portfolio. Related: Common Sales Tax Mistakes Small Sellers Make

Who Actually Pays the Back Tax

Sales tax is meant to be collected from the customer, but you did not collect it, and the state still wants it. For consumer sales the practical answer is that you absorb it. Going back to a shopper who bought a hoodie eighteen months ago for an extra few dollars costs more in support time and goodwill than it recovers, and many will simply refuse. When you absorb it, check how the state treats your receipts, because some allow the amount collected to be treated as tax inclusive, which lets you back the tax out of the gross rather than adding it on top. The difference in a large cleanup is not trivial.

Business customers are a different conversation. A registered business that should have been charged tax often owes use tax on that purchase anyway, so a clear, unapologetic email explaining the correction and attaching a supplemental invoice is frequently paid without argument. It is worth doing for your larger accounts even if you write off the long tail. Whatever you decide, book the liability now rather than when you pay it, and expect interest to accrue monthly until the money is remitted, which is the one component almost no state waives.

Closing the Loop So It Does Not Repeat

Once each state is settled, the work shifts from cleanup to monitoring. Set a recurring check, monthly is plenty for most stores, that measures rolling sales and transaction counts by state against the current threshold in each one, and flags anything above roughly eighty percent so you register before you cross rather than after. Thresholds and the measurement period differ between states, and several have changed how they count in recent years, so treat the threshold list as something you refresh rather than something you configured once.

Then make the operational side boring. Register, note the assigned filing frequency, put every due date in a calendar with a reminder several days early, and file zero returns in quiet periods because a missed zero return still generates a penalty notice in many states. Keep the exemption certificates you gathered during the cleanup in one place with expiry dates. The reason this cleanup happened is almost never negligence, it is that nexus is invisible until someone looks. A small recurring check is what makes it visible. Related: Preparing for a Sales Tax Filing