Calculation is trivial if you are only operating in a single tax jurisdiction, maybe. Tax calculation software handles all the different tax rates for product

Calculation is trivial if you are only operating in a single tax jurisdiction, maybe. Tax calculation software handles all the different tax rates for product categories your business sells, changes in city/district/federal tax rates, brackets and min/max caps, sales tax holidays, etc. It costs money to maintain those databases. Saying it can be done by any developer in a day is absurd.