I have an userform with 12 lines of 3 text boxes (36 textboxes total)

The first textbox in the line looks up for a costumer name and places a
balance in the third textbox. The second textbox holds a payment that reduces
the balance (third textbox). I got that part taken care of.

My problem arises when I try to validate several data entry scenarios: the
same costumer making several payments, (up to 12) either in consecutive order
or not and being able keep the running balance and place it in the 3rd text
box of the respective line.
Any suggestions?