This letter can be sent to a customer after it has been a certain length of time since you first contacted him/her.
{Date}
To: {Name}
{Address}
{City, State, Zip}
Dear {Recipient},
It has been {length of time} since I first contacted you regarding {reasons for contact}.
I'm certain that {name of company} can offer you exactly what you and your company need for {what your company supplies}, and I will call you on {date} so that we can discuss it further. Should you have any questions or concerns before then, please don't hesitate to contact me.
I look forward to hearing from you.
Thank you,
{Sender}