Couldn't you have two goals? One for basic and one for premium.
Basic:
- example.com/start_basic
- example.com/enter_payment_details
- example.com/thank_you
Destination URL: secure/submitorder (or whichever is the basic destination url)
Premium:
- example.com/start_premium
- example.com/enter_payment_details
- example.com/thank_you
Destination URL: secure/common/callback (or whichever is the premium destination url)
Unless I'm missing something. BTW, this would probably be better suited under StackOverflow Webmasters.