One time price and not plus everytime that quantity change

[jannisl]

Hello!! is there a way if i press check box for a plus price in the same product but not to plus is everytime that quantity change.. example

i have a product that costs 10 euros then i have a check box in custom fields with +5 euro that if pressed the price goes to 15 euros..
so if i have the same product but i want 2 of them the price is 20 euros... then i press the check box +5 and the price goes to 30 euros... i dont want that.. i want the price goes to 25 euros...
means that the +5 price stays the same.. any suggestions?