Windows

I'm looking for a way to add a bill accepter payment option to a photo booth software without going into the source of the program. There are many photo booth programs out there but not one we found so far has the option we need, so we plan to bypass any program we choose. After the picture is taken, and the preview displayed, the photo booth program asks how many copies should be printed. After the number of copies is selected, and clicked print, we need the photo booth program to be interrupted and the payment program to be initiated, asking for the amount to be inserted in the machine based on the number of copies selected. The price needs to be discounted with each additional copy. After the correct payment is made, the printing should start, and the photo booth software should go back to its default mode. For initial 10% payment I'm looking for a project breakdown and the algorythm of the program. Budget is $300.