Yes, everyone needs to select an event regardless of their payment method.
That's the problem. I need to design the form and write the script in a way so that those who select plans a and b are not sent to PayPal. Only those who select plan c would go to PayPal.
I'm having a tough time figuring out a design which will allow me to accomplish this.
With the CGI module, you'd just check the value of the payment_mode param (or whatever you want to call it) and generate the appropriate response accordingly.