Project title: phxsummershowdown.com
I built a registration form for karate tournaments. In this form, the parents can select one or more tournaments for their kid(s). I made it so the first event had a static price, then additional event prices varied on what they were. When the whole form is filled out with all the desired tournaments, a Paypal button is enabled so you can pay for the events. Each time you clicked an event, it would be added to the paypal form so the events would be listed in the summary before confirming the payment with Paypal.