I've been pulling my hair out with this piece of garbage for about 2 weeks now. Despite purchasing the gold cart add-on, which qualifies for "premium support" I didn't even get a reply from the developers regarding:
1: It doesn't send shipping costs to Google checkout.
2: It's virtually impossible to work on due to poor coding.
3: It's a mess. Mixed up class names, method names and old redundant code all over the place.
This thing is a hobby - and a poor one. I've almost lost a client because of it and I'll never touch it again with a 10 foot pole.