Just pushed big update on shop module. I don't even remember everything, but here are few details:
Shipping options, with additional payment costs (currently only fixed cost shipping option available, but it would be pretty trivial to build more advanced ones also)
Uses cookies instead of session - I settled for 7 day cookie
ShoppingStepsMarkup.module, which renders 1-5 steps to completed order
Bunch of bug fixes (and probably lots of new ones introduced)
Most of the fields created are now system fields, so installing this will not pollute your fields & templates. (This doesn't mean you should install this on your favourite live site - don't do that [with any module])
Updated the readme with few (well.. 12 actually) simple steps how to get this baby up and running
Get it while it's hot: https://github.com/apeisa/Shop-for-ProcessWire
All the feedback and testing is greatly appreciated. Thanks.