Spiny Box Puffers? Hmm, I'd wait a bit, as it sometimes takes a couple days for fish to get acclimated to their new home. In most cases, they should eat. There is a chance, however, that they will not, and at that point, return them to your LFS.
normally it takes them a little while to get used to the tank. i would try live brine or ghost shrimp to get them started eating and then move to frozen or formula one.