jdv, the shop code is in your Post.php file 4 times. Try the attached Post.php file, it should solve your problem.
It only goes to zero. Would be cool if they could go negative
They can't go negative, because I've set up the database fields as unsigned (this means that the range is from 0 upwards, and doesn't include negative numbers). If you want people's credits to go negative, you can edit the field in phpMyAdmin. Open the smf_members database in Structure view, and find the money field. Edit it and remove the 'UNSIGNED' attribute.
Does previous items on this site made by others work in 3.0?
Yeah, they'll work fine... SMFShop items are backwards compatible. There's a few extra settings in 3.0, which you'll find by looking at the test items.
Maybe credit for making a post with attachment? Just ideas.
That's an idea, I'll consider it