All packages that weigh more than 10 ounces (or are more than half an inch thick) and bear postage stamps for postage must be handed to a postal employee by the mailer, because of National Security regulations about the weight of "untraceable" packages (postage stamps can't be traced to the sender). 

If you are unsure whether your Box weighs more than 10 ounces, you can weigh it (if you have a postal or dietary scale), or take it to the Post Office to weigh it.

See current MediaMail postal rates here

If your package weighs less than 10 ounces (and is thinner than half an inch thick), no need to go to the PO!  You can mail it from your outgoing residential mailbox, or drop it in a blue mailbox. 

If you do need to weigh your package, or if you know that your package weighs more than 10 ounces or is more than half an inch thick:

• You do not have to wait in line if your PO has an Automated Postal Center
  • These are often available in the PO lobby, 24 hours a day
  • Each APC has a scale; you should bring a table of current MediaMail postal rates (print this one) with you, although there are often postal rate tables taped to the APCs.
  • The APC allows you to select your postage and generate a custom metered "stamp"
  • There is usually a bin beside the APC into which you can drop your meter-stamped package
  • Because this can be traced back to you (through the credit/debit card used to make the purchase), it is okay to mail your package weighing over thirteen ounces this way, as long as it fits in the bin!

Once you have mailed your book, be sure to mark it mailed on your account, so that you get credit for sending it when it arrives!

Related Links:

Marking a Book Mailed
How much does it cost to mail a book?
What materials can be sent by Media Mail postal rate?
How to Respond to a Request
Getting/Printing the Address to send a Book
Wrapping a shipment for mailing
Using an APC for mailing books