Are Books on Tape also allowed and if so how does that work if you want regular paperbacks?

Yes, you can post audio books, both CD and tapes.  Audio books are worth two credits, so if someone orders an audio book from  you, you will receive two credits for it.  You can use those credits to request any type of book you want- including paperbacks, which only cost one credit.  Just be certain that your audio books meet PBS posting requirements- those can be found in the Help Center.

Welcome to PBS!

Lesley

Edited to add: Here is the HC section on audio book posting requirements:

Audiobooks

• Audiobooks must be originals--not copies! 
  
  • Copies are illegal.  Swapping illegal materials here will get you expelled from the club.
  • burned audio downloads are not permitted on the site--these could be legal (being the only copies you made), but since there is no way to verify this, they are not permitted.

• Audiobooks do NOT have to include the box that enclosed the individual cassettes/CDs
  • The audiobooks should be packaged well if the box is missing, to prevent damage by USPS en route
  • Audio CDs must be shipped in CD cases/DVD cases, to protect them from scratching, if the original box is missing
• Audiobooks must be playable to be swapped here, without more than two (minor) skips.