martinshoplandAs a PHP and WordPress pro, my advice would be to actually test and debug the functionality of this plugin before release, the more I work with it, the more silly mistakes I find, mistakes that wouldn't have got past even the most basic test cycle. If you want people to buy your upgrades you'll have to show that you know what you're doing with code first.
Pay special attention to your Google Checkout callback; your tax calculations, especially when using coupons; and test, test, test!
