Unable to test shopify new payment app extension in merchant sandbox store

I need help on one que from the support . We won a new payment extension with shopify which we migrated from HPSDK

with HPSDK we used have option called payment sandbox URL but with new payment extensions shopify has removed this option and we are unable to provide possibility of testing our apps in merchant sandboxes . How can we support sandbox testing for merchant.

We can not have same payment session URL supporting both PROD and TEST . We have different payment sessions URLs for each env.

As far as I know there is not a option to have different URL. Maybe you could setup a service that operates from 1 URL and then sends request to either your live or test URL in the background?

No cross domain communication b/w PROD and non PROD is not possible in my org. I wonder why shopify has removed this option . HPSDK has the option to set up two sets of end points for sandbox and PROD .

There are many features like this are not supported in new payment apps

Can anyone suggest what is the best approach for this

For example merchant-1 went live with payment app

Merchant-2 wants to install same app in their sandbox and validate , then only merchant-2 wants to go live .

I can not proceed with merchant-2 request since merchant-1 is already live , any changes to the app will impact the live merchant.

Please suggest how to proceed in this case.