I am assuming you want to use paypal with this form which mean you need to use an HTML form. Unfortunately Formscentral doesn't support calculations with HTML forms. This is going to make discounts difficult. Depending on your exact needs you might be able to get away with using different fields for the discount. Example. Item one is standard room rate. Item two might be the same room with a AAA discount applied. etc. You will have to play with it to see if you can get it to do what you want.