From your description nothing seems out of the ordinarily
So I'll just give you a overall guide to take care of them
1.when buying them try to get the ones that are most active and looks more healthy
(Or the ones that are eating well if you get to see them being fed as
@GaryE said)
2.when setting up a tank for them(before buying them)try to do as natural as possible (with lots of plants, driftwood and preferably a sand substrate)
Also get the water parameters right specifically the temperature cuz pea puffers are tropical fish species and they require around 25-28°C
3. don't try to feed them in the first day give them time to settle down without disturbing them (try to keep the lights off)
4.once they settle and starting to explore the tank give them a tiny amount of LIVE FOOD (baby snails are the best but bloodworms/tubifex worms/vinegar eels are fine too) make sure to watch if they are eating if they aren't give them another day
5.after they start eating live food you can progressively move to frozen and finally to a good pellet or a flake made for puffers
(Make sure to give them some baby snails as treats for few times a week or you can get some snails to populate the tank which is even better)