All interstate transfers must go thru a licensed FFL for the person receiving the item. You the seller need not use an FFL to ship as it would add more transfer fees unless the dealer in the other state is being an ass and only accepts private transfers from other FFLs.
You can ship via any common carrier. UPS, FEDEX, or USPS. You should declare the item at the counter and insure it accordingly. Do not put any markings on the package indicating it is a gun. Most carriers will require you ship overnight, next day, etc. so fewer hands will supposedly handle said item. Personally, I have great luck with USPS and they are available everywhere unlike the other two. I have heard many horror stories about stuff being stolen, particulalry with UPS.
The buyer should make arrangements with the dealer to accept the package. I usually ask for a file copy of the receiving FFL's license for my personal records. Include a letter identifying yourself, the make, model, serial number, caliber of the gun, who it is being transferred to, extra items included (mags, parts. etc) and contact information for yourself and the buyer. Few dealers, for state tax reasons, also require a value be declared.
The process overall is not bad.
HTH