This letter informs your online service that you are canceling for reasons which you may want to keep to yourself, or not. Probably not.
1234, Main Street
Boston, MA 02123
01 April, 2005
<Recipient Address Goes Here>
I became a customer of <company name> <time you became a customer>. While I have found your service useful, I must inform you that I am cancelling my account, effective immediately.
I thank you for your services in the past. Please cancel my account immediately. If you have already billed me for this month's service, <disposition of bill>. Thank you for your kind attention.
encl: <List of enclosed items goes here>