Dogtown's write up is very detailed and (to the best of my knowledge) 100% accurate....
The only thing I would add to it is that UPS Store outlets are privately owned franchises and as such, the owners make their own rules in addition to the corporate franchise requirements. I've yet to hear of a UPS Store that will ship a gun.
On the other hand, all Fedex retail outlets are corporate owned and play 100% by the corporate Fedex rules.
Both UPS and FedEx have numerous 3rd party retailers that will ship packages, such as Office Depot, Staples, etc. Just because a business has a FedEx sign outside does not mean its a company owned location. ONLY FedEx Office locations are company owned.
And while you can legally ship a long gun via. USPS without being an FFL, don't be surprised if the minion behind the counter gets a scared look on their face and in their ignorance defaults to the "you can't do that" answer. I have avoided this by declaring the package to be "machine parts"
And that's a felony.
If asked by a postal clerk "whats in the box?" You have to respond truthfully.
That said, in twelve years of being a gun dealer, shipping a least 600-800 gun shipments.... I''ve never been asked a single time by a UPS/FedEx or USPS employee "Whats in the box?"
As a licensed dealer, I have to submit a USPS Form 1508 "Statement of a Shipper of Firearms", but only when shipping a handgun. Rifles? Nope. Shotguns? Nope. And I don't need to do anything but tape my Form 1508 to the box when I drop it through the little hole in the wall at the local PO. No human contact means no human contact.
I just don't get where people think you have to "declare" anything.
You can't do it on the UPS/FedEx/USPS websites when you print a shipping label and no law requires you do so unless you are shipping interstate to a nonlicensee....a felony except for a couple of very narrow exceptions.
Lying about the contents, ie "machine parts" doesn't change the fact that the contents is a firearm. Do that with UPS/FedEx the most they'll do is deny any claim for loss, damage or theft. Do that with USPS and it violates federal law.