warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, '' was given in /www/public/includes/form.inc on line 372.
warning: array_shift() [function.array-shift]: The argument should be an array in /www/public/sites/all/modules/amazon_store/amazon_store.module on line 1602.
Seems to be cached. Disabling cache fixed this but apparently the cart is cached for anonymous users so anything you add goes to an empty cart. Gave up on this as a patch for this did not fix my issue in addition to the fact it requires some theming.
warning: Missing argument 1 for _system_themes_access()
was the effect of upgrading the amazon_store to 2.x-dev without running cron afterwards.