Good breakdown. For the Canada leg there is one gotcha worth calling out: the tax follows the buyer's province, not the seller's, so your province lookup is only as good as the address you resolve it from. A second one is product categories. Manitoba charges 5 percent GST only on groceries but 5 percent GST plus 7 percent RST on general merchandise, so the same buyer in the same province gets two different totals depending on what is on the invoice. And Quebec's 9.975 percent QST is calculated on the price before GST is applied, not on the GST inclusive amount. If you want to offload the Canada portion instead of maintaining that table yourself, TrueNorth API has a calculate tax endpoint covering all 13 provinces and territories with 41 product tax categories, plus an address validation endpoint for the buyer location part.