In addition to the above suggestions, I'd add that it can be a good idea to express your own responsibility for any miscommunication.
Often the reason you end up writing a letter to a supplier is because there has been a flaw in the flow of communication between you and the supplier. Recognizing the problem as one of communication and one which you may have helped to create can go a long way to letting your supplier know that you simply want to be "on the same page".
You have excellent suggestions above. I would add that if you are expecting the supplier to do something specific (for example send you a replacement product), that you also set a reasonable time frame in which you expect this to happen.
Also, make sure you include any key details. For instance, if you spoke with someone over the phone, you will want to include dates and names of the people you some with. Include dates and any other key information about the problem. Remember to use business format, formal writing, and proof read. In business format, you will want to start with today's date at the top, followed by your address, the business address, and then the person (or position if you do not know the person's name) you are addressing. Here is a link to several writing tips as well as an example of a business letter format. http://owl.english.purdue.edu/owl/resource/653/01/