When a customer makes a purchase using 2Checkout, and the order is finalized, the order is not captured within Shopp.
This could technically be due to a variety of issues. However, all should be fairly quickly identifiable.
Some of these issues could be the interference from the theme or another plugin, the return URL, or the type of return being made for the customer.
Step One: The first step should be to disable any other plugin aside from Shopp and revert to
Step Two: Check your 2Checkout settings. Go to 2Checkout.com->Vendor Login->Account->Site Management. Make sure the correct URL is being used for the return. This is particularly important if you’re url is something other than /shop/. Also make sure the return option is set to “Given links back to my website”.