The way it has always worked for me is that if I have any OBC "on file" with princess (I don't book through a TA, normally) it uses that OBC first when trying to pay for an excursion. You can "uncheck" the box that says "use OBC" if you like, but that box has always been checked by default. If you don't have enough OBC to cover the excursion...then it proceeds to allow you to pay via PayPal or credit card. That's been my experience for several years now.