Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
This letter will serve to confirm our agreement regarding your purchase of, and payment for, products from our company.
We have agreed to ship the following products to you:
[products]
You agree to abide by our printed terms and conditions, and you additionally agree to pay the alloted price: [price]. The payment is due by [date].
Please sign and return a copy of this letter.
Very truly,