Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of client],
We have been retained by [name of client company] to conduct a short survey regarding [reason for survey].
[name of client company] would like to ask for your valuable input in improving their services and products. We have enclosed a very brief questionnaire that would provide them with important information regarding how you feel about their services.
We would very much appreciate if you would take a few minutes to completed the enclosed questionnaire and return it to us. For your convenience, a self-addressed, postage paid envelope has been included.
Thank you for your time.