When you send out a gift, usually there is a card or something that accompanies the gift as well. Sending out a letter accompanying a gift is a small token of appreciation and gives them something to read with the gift. You can make your customers happy and show an extra level of appreciation with a gift with a letter.
Explain that you have enclosed a letter with the gift.
Tell the recipient to enjoy the gift and wish them well.
Dear Mr. Tonel,
I have enclosed a letter with your gift as a token of my appreciation for all that you have done for this company. Happy five year anniversary and I hope you have many more happy years!