We did this and we certainly got our $$$ worth. We came home with one of each of the ponies, 2 outfits for each, slippers for each, a juke box, an outfit for each of the 5 ponies my daughter had at home already, brushes for each, barrettes, and more....
The trick, like mentioned before, is to use all of the space. Dress the ponies before you put them in. Put a pony in the bottom of the box and fill in the spaces between their legs, by their heads, etc and put another pony or hard item in and fill the spaces again. If you have a bed or anything with an open space, fill that space up as well and set it in the box with that filled space down and continue to fill in the gaps. As long as you can close the box correctly, that is all that matters....well, and not have it tear at the seams. My daughter's box bulged and they did not care because it closed.