Each library mailbag will also have a return label. Keep the bag and mailing label in a safe place until it's time to return your item. Once you are ready or the items are due, simply place the return label in the window on the library mailbag and put them in your mailbox for the postal service to pick up.
You must return all your items together in the same bag they came in. The postage is paid for by weight so it's important that you keep the items together when returning them through the mail.
Once your items are returned new items can be mailed.