just to let you know there is a special rate for books and such things as cds and dvds i believe it is lower than the regular shipping rates ask the next time you mail books had to go find this for you this is from the USPS website:
Media Mail
Send your printed or recorded material using this service.
Media MailŪ service is a cost efficient way to mail books, sound recordings, recorded video tapes, printed music, and recorded computer-readable media (such as CDs, DVDs, and diskettes). Media Mail can not contain advertising except for incidental announcements of books. The maximum weight for Media Mail is 70 lbs.
There are presorted rates available for bulk quantities of Media Mail (minimum quantity is 300 pieces).
A barcode discount is available for Media Mail.
Rates are based on weight and size.