Changing Payment flag after initial data entry.

I sell world sim cards and I need my order module to automatically create the billing based on the options that a customer chooses for his trip. After the order is entered I will bill the customer for the amount that was calculated by what he choose. If the customer comes back into the system and makes a change in the order that will cause the amount billed to be changed , how do i setup a flag that will compare the amount billed to the new amount and then set the flag to rebill, refund, or no need to bill again.

Order Total: $ 888.88

Billed Date: MM/DD/YY Payment Flag: (Bill, Paid, Rebill, Refund)
Billed Amount: $ 999.99

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