To improve your awesome idea I would suggest that you include an option that allows to download a PDF (or RTF to allow for modifications) with the text and the postal adress of the service. Theoretically, you could even extend your service and send it on behalf of your customer via mail for a few bucks (euros). That would be even better!
Edit: A similar service offers aboalarm.de (in German unfortunately) that ends your magazine subscriptions on behalf of you via mail for, I think, 2.99 EUR. This saved me time to get the mail adress, set up the letter and run to the next postal office.