2. We have at an earlier point created manual invoices in Zoho Books. (old balances for the customers at the time of converting over to Zoho).

3. For some customers, we are collecting payments for their subscriptions fine in Zoho Subs from their registered CC, but we want to use their CC registered in Zoho Subs to collect the unpaid balance from the invoice created in (2).

4. When in Zoho Subs, I select the old unpaid invoice (the invoice created in Zoho Books), and then click "Collect Charge".  I then get the warning "Select a paymnet gateway to make online payment".
How do I do that for this invoice? Stripe payment gateway is set up both in Zoho Books and Subscriptions now.

(5. When I try this in my TEST organization, I AM able to collect in Zoho Subs an invoice I have created in Zoho Books)

Happy for your input on this.