Finding And Selecting Last Non-Zero Cell In A Column

I putting together a spreadsheet that applies Payments (as they come in) against the oldest open invoice. As payments come in, old invoices are closed out. An aging is done and late fees are applied.

I have made a macro that inputs all of the information of the invoice in a list. However, when a payment comes in, I am trying to write a macro that:

1) in column G - Finds the first non-zero balance
2) Applies the payment amount to that open invoice. If there is additional funds left over after the satisfaction of that invoice, I would like the payment to be applied to the next open invoice until all the funds of the payment have been drained.

I am having so much trouble trying to even just locate the first nonzero balance and select it.