I am selling a long gun (receiver) to a individual in another state. I know it has to go through a FFL. It has been a few years since i did this. His FFL is OK with getting it straight from me which is good. The FFL is a regular retail store. He email me the info, phone number, name and address. Google shows the store with all the same info. Looks legit to me. Do i really need a copy of the FFL in the box or anything? Can i just drop the receiver in a box and mail it to the gun store with a letter of who it is from and my info?

No a copy of your drivers license is fine. The FFL has to show where the firearm was acquired from