When selling into the US makes you owe tax there
Economic nexus explained for Canadian businesses — the thresholds that create a US state filing obligation, why they differ by state, and how to see the crossing coming.
What changed
Until 2018 a US state could generally only tax sellers with a physical presence there. South Dakota v. Wayfair ended that: a state may now require you to collect its sales tax based on economic activity alone. You can create a filing obligation in a state you have never visited, using a warehouse you do not own, from a country you do not live in.
The thresholds are not one number
Most states use a dollar figure, commonly $100,000 of sales into that state in a year. Some add or use a transaction count. Some measure gross sales, others only taxable sales, and others only retail sales. Some look at the current year, some at the previous one. Treating it as a single national number is the mistake that produces a surprise assessment.
Marketplace sales usually do not count the way people expect
Where a marketplace facilitator collects and remits on your behalf — which Amazon does in every state that requires it — that tax is not yours to collect. Whether those sales still count toward YOUR threshold varies by state, which is why the same revenue can create an obligation in one state and none in its neighbour.
Crossing it is a date, not a surprise
The obligation begins when you cross, and registering late means owing tax you never collected out of margin you already spent. The whole value of watching the figure is that registration becomes a scheduled task rather than a letter.
How SeroBooks handles it
Sales are counted against each state's own test rather than a single threshold, and the figures carry a review date — most recently 28 August 2026 — because thresholds change and an unmaintained number is worse than none. Canadian and American books stay separate systems throughout: different tax, different payroll, different year end.
Tax handled per line, per province, automatically.
The place-of-supply engine inside SeroBooks does this all day.