First thing to remember is that you will have to take the gun to a real UPS hub or a Fedex hub. Places like UPS store or other vendors will not take handguns, for the most part! Bring a copy of the FFL with you to the shipper.
Next make out a bill of sale of sorts
Shipped to: IE the FFL holder receiving the hand gun
for the purpose of delivery to the actual buyer.
List gun description of gun including Serial Number.
List any accessories included with gun, ie scope bases and rings?
Any documentation of work done?
List agreed upon return conditions Such as say a three day inspection and if returned the buyer assumes all shipping costs?
Perhaps put a copy of Driver's License on the Bill of Sale so the FFL has an accurate accounting of the source.
Do you have a FFL holder that has a UPS account? Perhaps you can get him or her to ship it rather then messing with going to a UPS or FEDEX hub? Or they can ship FFL to FFL via USPS that even insured would possibly be cheaper......