Using gift registry how does someone pay for the items being sold

You need to complement it with a payment tool in the form.

User guide: How-to-Pass-a-Calculation-to-a-Payment-Field

A much better example of this method was used in the Ticket Purchase widget demo form https://form.jotform.com/60130611186949

Try cloning the demo form so that you can better review it on your Form Builder.

User guide: How-to-Clone-an-Existing-Form-from-a-URL

Let us know if you require further assistance.