I use US Postal Service Priority Mail for the bulk of our shipping requirements. It's proven to be very reliable and you do not have to put up with behind the counter bullshit with regards to someone's personal agenda. It is perfectly legal for a private citizen to ship a firearm (rifle/shotgun, -pistols are a little different) across state lines and not have to provide any sort of additional documentation.
The only requirement is that the transaction has an FFL present in some capacity. Either coming or going. That is a BATF thing, the USPS aren't the police. If you get friction, you demand to speak with the Post Master and you have your poop in a group. Educate yourself!
The DMM Revision 603 covers this.
A little known fact about USPS Priority Mail. It's actually FED EX for the bulk of the commute. USPS subs the work to them. USPS handles the parcel up to the regional disbursing office, then FED EX takes over. It's delivered to the next area where USPS again takes control.
IF you ever ship a gun via USPS and it goes MIA, contact the Fed Ex hub in Memphis, TN and be a persistent shit head until you get someone to go into the cage and physically look for the item. This happened once about 5 years ago. The label was torn off the box somehow. KNOW YOUR SERIAL NUMBER as it will be asked. They WILL breach the seal and verify the contents. It's a good idea to package things well, but know that when looking for shit they won't be nice. Making the SN easy to find works in your interest.
This is a very rare occurrence. It happened to us once as stated. I spend close to $35K annually on shipping. That's a whole lot ah' stuff...
UPS can suck a dick far as I'm concerned. We use them only as an absolute last resort. I've had terrible experience with them over the last 20 years.
Good luck, hope this helps.