The real secret is that DB uses ordinary mail, that is, airmail, rather than a more expensive next-day-type delivery service. That seems to happen less and less in the U.S. even for fairly routine mailings.

For example, Morton's sent me a gift certificate in response to a "complaint" and I was looking for it in my mailbox; instead, it arrived a week later in the form of a Fedex envelope on my porch. That was overkill.