1) Make sure any link in the buy now button is removed
2) Make sure there is nothing in your amazon affiliate code either
3) If you have the latest version of MyBookTable the you have Paypal as an option. Just put your link in there and make sure all other button links are not added.
4) If you don't have the latest version of MBT then you can still put your button code for paypal into the Book Details under the Text version. See example.