Pretty cool! Can't be more simple. I don't know much about website programming but isn't it possible for website's to sometimes see where you were just browsing and thereby get the address?
It uses SSL, which should be good enough. The only computers that should be able to see the URL are yours and the server, and the server already has the money. Certainly it's possible that some specially-written malware on your computer could monitor for accesses to this site and steal the URL, but it could do that with the name and password of any e-wallet service, or any private keys stored on your own computer.