Oh my, that tax calculation does get complicated in the general case! An order could contain a mixture of products with different tax rates (perhaps some taxed, some not) so simply recalculating the tax on the total doesn't work. Then there's the added complication of multiple tax rates being applied.

For my client, it's much simpler. A cart can include only one product and the (single) tax rate applied to that product is applied to the order's sub-total.