the quantity button

Hi, I want to be able to add different prices to the quantity option on my order forms example;
my main product is €29 and each additional copy is €12 each the problem is at the moment I have a custom made add extra copies but if some one uses this for two copies then see's the quantity option beside the add to cart button hits then the amount due doubles and they abandon the purchase, Alternative is can that quantity button be disabled. Thank you for any help. my web site is www.allcertsireland.ie

version 1.5.1.3

I was able to sort it by using the discount option eg.first birth cert is €28.00 and two extra certs are €10 each = €48.00/ this by 3 =enter the unit price at €16