Four options -
1) Envelope with Certified Mail tag and Insurance tag. The Certified Mail tag will make sure someone it is addressed to will have to sign for it.
2) Registered Mail with Insurance - You'll have to seal everything up in front of the mail person, but your mail is kept locked up and has to be signed for every time it moves from point to point.
3) Express Mail Flat Rate with Insurance, it will be delivered in one day and will have to be signed for.
4) Priority Mail with Insurance - 1 -3 days in transit, insurance value will cause them to make it signature required.
Options 2 or 3 are the most secure, but may cost $30 - 40 to mail.
The insurance cost is about a buck per $100 of value.