Here is an interesting page on Ego4u.com that generates business letter in English. I have found it by chance, and tried it out. If you visit the page, you will see the following templates from which you can choose:
- Invoice or Overdue Notice
- Dispatch Note and Acknowledgment Receipt
- Order Information
- Offer, and
- Enquiry
After the click on the template you will see common parts of the business letter (salutation, the first paragraph, subject etc.), with several options, followed by spaces for information to personalize your letter.

Created letters can be exported in textual and rich-text format (RTF). Here is an example of completed letter:

Letters auto-generated from templates are interesting and useful mostly as an illustration of the form. However, it is better to learn how to write them and improve your correspondence, which is, after all, business-like and professional.