Yeah honestly i would make sure it's nice and warm, but holding for 3 minutes immediately after eating COULD stress him/her out enough.

I work at Petsmart and can honestly say that if you feel his health is deteriorating, return him. They were selling them half off probably as just a random sale to clear out inventory. Also Petco is required to pay whatever vet bills are due and if they are anything like Petsmart(which they are), they will let the vet do all the work.

I would just call petco and let them know of the circumstance ASAP, that way if it does go downhill, they are aware. Make sure to tell a manager also.