People who order stamps online?
If the problem is writing the letter or stamps, then why not just send the user a postage-paid envelope, pre-addressed to Chase, with a generic letter that she can fill in with her personal information and deposit into the mail?
You should now be wondering how this "service" can be "free". As someone else has figured out, this is a company that wants lists of Chase customers to which they can market their consumer arbitration-related services.
This is interesting since the whole point of opting out here is that consumers want to avoid arbitration and reserve their rights to sue.
Other websites are offering a more sensible solution for those who cannot be bothered to write a letter: templates for a simple letter a customer can just print out, fill in her information, sign and mail.