I just got mine in the mail today and they are awesome! They are only available from usps.com, post offices are not supposed to carry them, but they are free with free shipping.

Now it's ALWAYS going to be cheaper to ship media mail than Priority Mail, at least for anything that would fit inside one of these. But if you ever feel extra generous and want to get a book to someone quickly, there is now a relatively affordable way to do so. 

It will vary by book size, of course, but you could fit about 4 mass market paperbacks in one of these, or 2 larger trade paperbacks. So if someone ordered 3-4 small books from you, you could fit them all in one of these, they get them in two days, and it costs you about $5 ( you can save a little $$ by getting the postage from usps.com). 

I'm definitely not trying to push anyone to use Priority Mail over media mail or to spend more money than they need to, I just want to throw this out there as an option :) 

If you ever ship books overseas, this can be a big money saver. I do a lot of angel mooches on bookmooch, that's why I am pretty excited about this, but it could have a benefit for PBSers if you ever want to be extra generous.

Shipping paperbacks overseas is probably cheaper using International First Class rather than Priority Mail.

Is International First Class cheaper than Domestic First Class?  Otherwise Media Mail is cheaper for any book larger than 7 oz.

For a single small book, it is cheaper to ship first class international. Depending on the weight, when you get to 2-3 books, the flat rate becomes cheaper (overseas gets expensive FAST). You can definitely fit enough in the flat rate envelope to make it worthwhile. I always check the weight and compare prices before I ship :)

There is not media mail overseas. They do have M Bags, but you have to have a LOT of books for it to be worth it (prices start at about $37, I think you need about 5+ pounds of books to start to be worth it) 

BTW,  Mail to APOs and FPOs is not considered international.  It is US mail and you pay only for distance to the port.  For European APOs it goes thru NY and then gets taken by the military to its destination.