Are you by chance ordering an XL sized book? That is known to present problems.
If you can't create a PDF file of the book as leonieDF has suggested then boot into Safe Mode, Mac OS X: Starting up in Safe Mode, and try to create the PDF again.
If you can create a PDF file while normally booted then boot into Safe Mode and order the book from there.
NOTE: another user today had the same problem with an XL book and was unable to order even from Safe Mode. They resized the book as I described to them in this topic:cannot upload iphoto book