Ecommerce module comprise of many modules dependant on each other. There is an error during the shopping 'checkout' process, something like:-
Invalid argument supplied foreach) /file_directory...contrib/ecommerce/store/store.inc

The problem is we have not enabled the 'address' and/or 'shipping' modules.

After we enable these modules, the buyers will have to include their shipping and billing address. Only then the error will go off.