Depends on how you want to sell them... I think if it's media mail you provide your own or wrap them. I am not aware of any packaging from the PO that doesn't cost you.
I talked to the post office a couple of days ago about media mail. It takes 5 to 10 "working" days on the "average"... which means don't count holidays, Sundays, or days when the delivery person can't make it there. We had a few people here in the Tenn valley area that didn't get mail for a few days..Now, the average can be longer.
Getting a confirmation cost about .80cents more and doesn't get it there any quicker. I sold several books at a really great price compared to what I have seen posted for sale here.
The other thing is that first class can sometimes be cheaper.. check the online PO for rates..