Zumbox and ECM are very different.
With ECM, you purchase a mailbox and get a real postal address in many destinations (USA and others). With every letter that is sent to your address - you get an email notification with a scan of the envelope. You can instruct ECM open and scan the content of your letter or you can instruct them to recycle it instead. It's a very convenient service because it saves you the need to handle paper mail.
Zumbox offer a new marketing approach. They try to get people to sign up and try to get marketers to send mail to those who registered. I guess their ultimate goal is to render the USPS worthless.
That's a nice and simplistic interface however I've been using PostalMethods (http://www.postalmethods.com) for awhile and they offer an email interface and a Web Service API (http://www.postalmethods.com/postal-api) for a cheaper price when sending regularly. They also support postcards.
I think Dustin's service is most suitable for occasional letter senders while PostalMethods is most suitable for businesses wanting to automate their mailing.