Short write‑ups of problems we hit running a real commerce operation and what we changed in the code because of them. No product announcements; just the reasoning.
Selling across Europe and the UK means every order has to answer one question before a price is final — and the answer depends on two addresses, a postcode, and whether a VAT number is real. How aiku validates tax numbers against VIES and HMRC (with retries, rate limits and an audit trail), and the decision tree that turns a shipping address into a tax category, Canaries and Madeira included.
For years every line we sold carried the same VAT rate, so one rate per order was simply true. Then the range grew to include tea — zero‑rated in one market, reduced in another — and "which rate" became a question per line. The answer was not "read the product's tax code": it was presets instead of codes for staff, tax versioned like price on the historic line, frozen once sold, and a migration that re‑rated the open baskets to the penny.