Welcome to YT - You have come to the right place.
I really felt relieved to hear that you will do everything in your power to keep the pup.
Like someone else stated, I COULD NEVER RETURN A PUP. He is an innocent soul and if you return him, sorry to say, but something very, very bad could happen to him. He deserves a chance at a happy life.
If they gave you meds, I am sure they suspect he has kennel cough, which is curable. It may take some time to run its course, but with meds and vet care he will survive. Just make sure to go to the Vet ASAP and get a diagnosis, b/c there is a possibility it could turn into peunonmia (sp?).
I can't wait to see pictures of the little fella
And again, please DON'T Bring him back, I fear for his life if he was returned to that awful, uncaring pet store hell.