design problem

Hi, I wrote a Access program for my friend. I used a payment table to
track which customers paid. And there is a paymentID. I link this to
orderID. One can make several payments to one order until it's 0.
Now, some of his big customers have credit. They can put orders, then
at the end of each month, they make a big payment for several orders.
They want a form to handle this. On the top part, there is the amount,
and on the sub-form, list all the orders made by this customer, with a
check box to fill up the payment and reduce the total amount on the
main form.
How should I do this? Can I use my current data structure or should I
make some more tables?
Thank you for your help.
Regards, Ming