Hi

there is another bug related to shipping tax and coupons but this may not affect you with using DISPLAY_PRICES_WITH_TAX.

I am however currently reworking some of the calculation logic, as currently the way code handles the difference between DISPLAY_PRICE_WITH_TAX = true/false is a little convoluted (as you can probably tell from the problems you encountered)

Hopefully I will have that done in a day or few.