The My Little Pony fill a box is in Once Upon a Toy Store (in the back of the store). It costs a little bit less than the Potato Head box.
You go around and collect the princess outfits and all the accessories that go with them. There are ponies at the botom to choose also. I was able to fit a pony, bed, armoire, teacup, clothes and all the other accessories into one box. I started with the big pieces first and then added all the outfits. As I was finishing up a CM was helping another person and asked the little girl if she wanted the bed or the teacup as you can only fit one in. Not if you can pack really well!
You can't get all of the Potato Head pieces in one box, but you should be able to get all of the My Little Ponie pieces into one box (unless they have added more since September).