Here are some suggestions on how to do a book collection.
Get a box about 18 x 4 x 12. Make sure the box does not say “Priority” or “First Class” or they will cost much more to mail. In the United States, the most affordable way to ship is from a United States Post Office. Mailing can be expensive so doing smaller packages spread out of time, is easier to do.
USPS Customer Declaration Form CN 22
USPS Customs Declaration and Dispatch Note CP 72
the forms do changes, so check with your local USPS.
You can ask people to stick $1 with the books they drop off. This will help pay the postage.
These forms are available at the United States Post Office