When developers first add VAT to an ecommerce or SaaS product, the problem can look surprisingly small: Find the customer's country. Look up the VAT rate. Multiply the price by that percentage. Done. That works until the first real edge cases arrive. What product was sold? Was the displayed price VAT-inclusive? Which country actually has taxing rights? Was this transaction created today or six months ago? Is it a refund of an older sale? Which rate was used when the or...