I was looking for new ideas for my Certificates of Authenticity to include with prints I sell, and found this site.
Very interesting ideas about additional information you might want to include on your certificates.
I enclose certs even if the piece is not a limited edition. It gives people information about the artist, the original art, and perhaps even the paper the print is produced on.
http://www.worldprintmakers.com/english/certauth.htm