IMO pack it all up and print the label at home and just drop it off at the post office or UPS drop off location. If they ask what's in it, say curtain rods.. Although it is NOT against any rules to ship an air rifle with either company, the people at the desk will freak out if you say the word "rifle" or "gun" no matter what is in front of it. UPS Store is actually privately owned franchise and not owned by UPS, they can and will refuse to let you ship it through them if you say "air rifle" under their own policy.
With USPS you literally just walk up and set it on the desk and say "prepaid" and they say "thank you, do you want a receipt?". It's that easy. You don't even have to stand in line usually, usually there's a small counter on the very end that you can walk up to and drop off prepaid stuff.