If you aren't a FFL UPS and FedEx will not ship a firearm, if they know. They implemented that several years ago. There are some companies, Bud's(?) and now Gun Broker have something worked out where you pay them a fee and ship on their FFL somehow. Haven't used it.
I took a long gun to the PO to ship registered with well over $10K insurance. I was talking with the person at the counter and said it's the safest way to ship, "it's locked up in a special cage every night, signed for, etc, right?". She just laughed. she went to the back and brought out a canvas bag. She said if it didn't fit in the bag it didn't get locked up. No way was it going to fit in the bag. It just gets left on the floor by the cage. And also know if you ship USPS registered, it could take weeks(!!!) to be delivered. And tracking is not reliable on registered.
USPS regs also say an individual cannot ship a pistol or a firearm that is "concealable".
Also, you better check the regs because I think you are required to declare it is a firearm when you ship. And don't expect them to pay any insurance if it is lost/stolen if you don't declare. You will be lucky to collect even if you do declare and the steal it.
If you go private insurance, IE Collectors, be careful what you tell them if you want to get a policy. If you say you want $X00K in insurance to cover when I ship guns I sell and some for my collection, they probably won't write you a policy. Ask me how I know.