It's good to have a letter with your name and address on it so they know who you are. Here is a standard letter form to go on. Dress it up as you like.
Dear Sir or Madam:
Enclosed please find a signed copy of my Type 03, Curio and Relics, Federal Firearms License. Please keep this on file so that we may do business in the future. Also,if you would send me a copy of your latest catalogue or sales flyer, and place me on the mailing list for same, I would be most appreciative.