Virtuemart customisation

I need someone who can do some customisation work on virtuemart. What needs to be done is the following: I use the custom attributes hack for Virtuemart 1.1 ([login to view URL]). What i now need done is a way to ad a price (or hidden product) to the cart depending on a choice made in the custom attributes hack. When i now try to add a price to a field, the amount gets multiplied when the customer orders more than 1 of the same product, the art of products we sell are typical the kind of multiple product of one type, the price shouldn't multiply, but only be added once to the order. To make a little bit clear for what it is going to be used for a explanation of our products: We sell greeting cards for weddings e.o., our costumer selects a card, and puts in a text that he want to print on the cards. Now the thing is that when the costumer selects to print a text on the card, an amount of 55 euro has to be added to the total price of the order. So it doesn't matter if the costumer orders 35 of 150 cards, the amount for printing keeps te same.