Dear, I am surprised that Shopify does not provide such basic functionality. It happens that by mistake, I manually marked an invoice as paid. To my surprise, there was no way to reverse or change the status. I opened a ticket and was told there is no way to change the status.
As the product has shipped, I cannot cancel the order. Shopify gave me only the option to ask the customer to pay in cash (impossible, as it is an international customer) or via ACH. Since our retailers pay by credit card, the only option Shopify gave me was to use another application to capture the payment.
To avoid a manual payment transaction, it should be as simple as possible, with no payment processors involved.
If someone found another way, let me know; otherwise, I have to use another application to capture a credit card payment manually.
Regards
Martin