The below is what I have adhered to and had no issues what so ever, Sorry if it sounds soap boxish/preaching, but it works for me. These rules I have garnered by reading posts like this, websites and personal experience.
1. Always specify that the buyer pays for shipping. To and back if they don't go through with the transfer at the receiving FFL.
Unless shipping is part of the deal.
2. Ship it in a hard case. They are cheap. Tape the hard case shut inside the cardboard box the hard case comes in. It is easier to have a damage
claim settled if you shipped in an appropriate container. Make sure it can't be moving, that way it can get no momentum of the case is dropped.
3. Always ship on a Monday. That way it doesn't sit around over the weekend. The more it is moving, the harder it is for an insider to steal it.
4. Always insure it for the cost plus the cost of shipping. That way you can make the buyer whole if something happens to it.
5. Whether or not they are required to send a mail/email copy, it sure helps if you have it with you when you tell the UPS/FedEx/USPS agent that
you have the FFL with you and they can see it. They don't want to get in trouble either.
HOWEVER, after a recent go round with USPS losing, then destroying, ripping the shipping label off and having the package returned because of the return address. Then having to have the local newspaper get involved to have them settle the insurance claim, I will never ever ship anything USPS again. This was some Valentine stuff we were sending to my daughter and her other half at the time on Lackland AFB in Texas. Beyond the local staff of the post office (they are your neighbors) USPS personnel are next to useless unless you hold their feet to the fire.
6. Make sure everything is taped tight and no loose edges. The companies use tremendous amounts of automation and you can see the marks on boxes a lot. I have been shipped shotguns (back when I was restoring Ithaca 37 16ga guns for my own use) that I was surprised made it without being broke. Boxes had edges grabbed, crushed, ripped. You name it. The more secure it is the better chance it will get there.
One time I even built a crate when I was shipping one of my better quality 1874 Sharps replicas to the factory in Wyoming. The extra $$ for the weight was worth the piece of mind a $2200.00 gun (for me that was a lot of money) would get there in one piece, especially since I had to wait
16 months for the 1st one and over a year to have another rebarrelled (backlog).
7. Never ever have anything on the outside show it is a firearm. If you can take the gun down and make it look less like a long gun, the better. I always (if possible) take the barrel off shotguns to ship. Makes them half the length and less likely to get broken.
8. Make friends with the folks at the terminals. Be nice. They remember. I have never had any issues shipping long guns to anyone.