Better World Books
This is a for profit institution that donates part of their profits and promises that for every book sold they will donate a book to a charity organization. They are good at picking up the books from you if you have a substantial size donation. If you have what you consider a large donation, you can contact firstname.lastname@example.org and find out if they will pick it up for you. If you have a smaller donation, they have several drop-boxes in the area where you can drop your books off.