There is a psychological difference between receiving an email and signing for a piece of paper from the mail carrier. Email can be archived, filtered or denied. Certified mail leaves a federal record with a date, a time and a signature.
In practice that changes three things: the defendant can no longer claim they never heard, your letter becomes evidence, and the cost of ignoring you goes up.
It is not magic. It is friction — and friction is exactly what is missing when someone decides not to pay you.