I think the OP just needed a "fake" billing address to access the rest of the application form.
Correct. For some odd reason, the form asks for delivery and billing addresses at the beginning--without them, you cannot progress through the form--and then at the end, you're asked to input the credit card billing address a second time. The delivery address I put in was correct (and that's where the railcards were sent), and at the end of the process, my credit card was charged, so clearly the billing address went in correctly also. I got an email this morning confirming that the railcards were in the post.