This letter tells a company that you are so impressed by them, you'll be happy to give them a testimonial for their marketing efforts.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I'm writing this letter to make you a simple offer. Since I have been so impressed in the past with <company name>'s <product or service>, I would like to offer you my testimonial any time you need it.
I believe in promoting businesses I believe in, so whenever you need a positive customer statement for a brochure, an advertisement, a newspaper article or anything else, I'll be happy to provide one. It's my way of saying thanks.
Thanks again for your outstanding <product/service>.