Giftcard from MAP / original price

Hi,

How can we mange that the giftcard we are creating is always taking of the original price of the shopping card and not of the tiered or goup pricing.
Example: CVustomer has a giftcard of $20
Original price of shopping card is $100
Custormer is in a grouppricing of 25% and only has to pay $75 instead of $100
Customer only pays 75 - 20 giftcard = $55

We want the gift card substracted from the orginal price:
$100 - $20 = $80
$80 - 25% = $60
Customer pays $60