You're payment API doesn't have a sandbox mode? Maybe that will work? Short of that, maybe you could create a fork of your application for the designer that just returns payments_processed=true and doesn't talk to the payment API at all.
You're payment API doesn't have a sandbox mode? Maybe that will work? Short of that, maybe you could create a fork of your application for the designer that just returns payments_processed=true and doesn't talk to the payment API at all.