its not dumb, sending it via registered mail is dumb.
Certified mail requires a person to sign for the item proving it was delivered to someone not just left in a mailbox. It is for important paperwork, some bills, etc.
Registered mail is FAR more expensive and is for valuable items like cash, coins, jewelry, collectibles, etc. It is sealed with specific kinds of tape and marked with cancellation stamps at every seam so it is protected from tampering. It is also signed for every time it changes hands and kept locked in safes, etc. when not in transit. It is the most secure way of sending anything valuable. Its also the slowest and most expensive since there's a chain of custody all the way along. This type of security is totally unnecessary for a letter like this. Certified is the correct choice.