on the confrim page, you are setting the recordset to filter using the URL Variable UserID.
The checkout form uses the POST method though and the Select list is named UserList.
on the confirm page, open the bindings panel, click the plus button and select Form Data. In the form data tool, selct the checkout page. This will adds bindings on the conform page for the form on the checkout page.
open the server behaviors panel and click the plus button. Select WebAssist -> eCart -> General -> Set Session Value. For the trigger and value, click the lightning bolt icon and select the UserList form element. Set the name to UserID.
in the recordset, use the UserID session to filter instead of the URL variable.