My system is
AW Coupon Pro 1.1.0
The Virtuemart coupon system works correctly. I use discount system before taxes and transport.
I'm not able to understand what kind of logic AWcoupon uses. This is a pratic situation with a coupon with 20% discount:
Item: 295,00 + VAT 20%
(the total should be (295-20%)+20%VAT = 283,20).