I just reread the mailer, and it doesn't say anything about needing to register.

I've closed Chase checking accounts via secure message, so it should be possible to do the same with credit accounts, although I'm not sure. However, I'd call up anyway just to see if they offer you a retention bonus - it can't hurt. Also, I believe that the annual fee is able to be refunded if you cancel anytime before the payment is due for the billing statement in which the annual fee appears. I've done it many times with Amex, although I'm not sure if that practice is standard among card issuers.