In order to use fewer mail bins and save wear and tear on the books, here are some tips to follow when placing books in the mail bins:
Put bigger books at the bottom of the pile.
Turn books so they stack neatly in at least two stacks. Often times you can make 3 stacks.
Reorganize as you place more books in the bin so bigger always go to the bottom and smaller on top.
If one bin is really full and the other not, move some books to the emptier bin to create an even load between the bins (this is being kind to the staff who load and unload these bins as they are moved around campus).
Put some of the smaller books around the sides or between the stacks, vertically or on their side, to take advantage of that space and help keep the other books from sliding around and getting damaged.