The Shopping Cart automatically will default to an amount of U$100.

To pay the amount of your invoice simply enter the number of U$100 Units you wish to purchase. For example,  if your invoice is for U$700 enter "7"  in the Quantity Box  "QTY" and then click on the "Update cart". You will now see that the total amount billed will be U$700.  Then simply checkout to complete the last steps of the payment process.

 When you are ready to begin click the "Add to Cart Button'