I've sold a few books on Ebay and made the mailers from scrap corrugated board from boxes. I've sent as many as 4 books in one homemade mailer and 5 would probably be no problem. Media mail is CHEAP compared to other shipping methods.

https://www.google.com/search?q=homemade...UTF-8#kpvalbx=1

Edited to add: Yep, I'm cheap and would rather spend my money on shotgun shells than postage.