If you want to enable the customer to make a payment on invoice, you only have to create a "Payment-Methods".
Navigate in the restaurant manager (cloud.gastrofix.com) to the tab "Payment-Methods" ("Setup" --> "Financial" --> "Payment-Methods")
Name = Debitor (can be changed to anything)
Receipt Layout = Receipt
Total Turnover = YES
Cash Turnover = NO
Payment Type = Debitor (Only settlement type "Debitor" uses customer accounts)