I have not sold & shipped a firearm in some time, but my understanding is that long guns are fairly simple & easy, because USPS will ship them for just about anyone. There is a long, flat Priority Mail flat rate box (designed for mailing board games) that can be useful for long guns like AR15s that take down into small enough pieces, but you may have to put in a request to get one (most post offices do not stock them). I think the last time I used one to ship an AR upper it cost about $25.
Shipping handguns is where most folks have issues, because USPS will usually ship them for FFLs, but will definitely NOT ship them for 'Joe Citizen'. So you're stuck using FedEx or UPS, and I believe they both insist on handguns going 2nd-Day, which costs an arm & a leg. Plus, your average neighborhood FedEx or UPS franchise store will not accept firearms (or even gun parts, in my experience), so you have to find a corporate store. The last time I sold a handgun & shipped it myself, I think FedEx charged me $75 for the service.
Because FFLs can ship a handgun by Priority Mail, it's often cheaper to pay an FFL to handle it for you. The last time I did that, it cost me $45, which beats the heck out of $75, and helps support your local FFL. (Of course, my FFL shipped it to the wrong state, but that's another story! ;^)
As always IMHO, IIRC, FWIW, YMMV, etc.