Well if there ever is/already is a mailing system this is my proposal.
Ive always been sketchy about parcel trading because you dont know if you can trust the person. So how about this.
A player would send the parcel/mail but the player receiving the mail could only accept/get it if he/she sent their parcel. Than the other player would be able to see what is in the sent parcel before accepting it. Once both players accept they can receive their parcels.
Something along those lines.