Send this letter as a follow-up to information you had sent a client.
{Date}
To: {Name}
{Address}
{City, State, Zip}
Dear {Recipient},
Recently, I sent you a letter regarding {contents of the letter}. In case you did not receive the letter, I'm including the same information here. If you did receive the letter, and you think you'd like to know more about {contents of the letter}, please feel free to give me a call anytime. I'm happy to speak with you at your convenience and explain everything in further detail.
I look forward to hearing from you.
Thank you,
{Sender}