I'm selling a few products. I need to get some options at checkout (like Do you want to add installation services?) that will affect the price, and I need to reference a custom database table to do some complex price calculations to get a total order price.
I wanted to use getshopped.org's wp e-commerce shopping cart, but somehow override the checkout page to do what I need, then return my calculated price to wp e-commerce to finish the order up.
How would I do this? Would I use a hook? Should I change some method that calculates the totals (without losing my changes on the next update)? Or do I have to drop the plugin and roll my own completely? Thanks so much.