Then come back the next day to get them back. This is what happens:
- At the mechanic, you'll pay your bill. The mechanic will hand you back your keys and maybe give you a receipt detailing the work. Maybe not. You take your keys, start your car and go about your business.
- At the gunshop, you'll pay your bill. Then the gunsmith will hand you an ATF Form 4473, which you will have to fill out in order to get the permission of the Federal government to get your own gun, your own fucking property, back from the gunsmith. If the Federales's computer system is working (by no means a given), you can then get your gun back.
Another difference: You can tell your mechanic "I can't get away from work, so my spouse/co-worker/friend will pick the car up for me" and probably your mechanic will say "cool beans" (as long as the shop gets paid). Not so much for guns, you have to be the one to go down and reclaim it. Possibly a close family member can do it, but for anyone else, it'd be a "straw man" transfer and, if your local Federal prosecutor is a dickhead out to make a name for himself, both of you can end up doing hard time.