Add product attibute price increase only once

[JayTee]

Hi all,

I'm almost done with my shop but I have the following problem I can't figure out.

I have a product X it costs $10,- but to make product X I need to design something at the cost of $15,- or reuse a design from the customer's previous order.
So I did the following:

Create a product option group, Setup-fee.
Added "New design" and "Re order".
Ran the product option generator (have some color options as well) and added the $15,- to the New Design.

Now this works fine for 1 product in the basket. The price is 25 in total. or 10 when reordering is selected.

The problem is that is I add more then one item the price is calculated OrderAmount * (10 + 15). So for two products the price is 50 instead of 10 + 10 + 15 = 35. Now i don't mind making some extra money... but this is a bit to much ;-)

Any help is appreciated.