The checkbox is too light and/or too small. Too easy for customers to overlook that they need to check the box.  The way it's designed it appears to the customer they are authorizing the CC to be kept on file just by paying the invoice.   I know I can't customize this, but can it be changed.  Heck, I know the box is there and it's hard for me to see it, let alone a first time customer who isn't instructed that there is a box to check.  Even if you just added the words: By checking this box, I authorize XYZ to store and charge........