Below is an example of what your shopping cart could look like:

Pay Your Bill Online!

 

Here your customer could simply enter in the amount they would like to be charged and the invoice number they would like to pay off; once they select "Continue to Payment Form" their browser will open a new window allowing them to enter their billing name and address, as well as, their credit card / ACH information. If the customer enters their email address they will also be sent a receipt.