The most secure way to ship via USPS is with
Registered Mail. A legal chain of custody is maintained from pickup to delivery. In the event that such a package is lost, USPS is able to identify the individual who last signed for custody of the package and take action if necessary. Registered Mail is kept under lock and key by each employee who signs for custody. The cost is dependent on the value of what is being shipped, but it does include insurance.
FedEx has a signature release card on file for me at their local office, so my recent Arc order was left for me at my home. But, in general, shippers can request an in person signature. The release doesn't apply for those, requiring a 90 mi round trip for me to pick them up (don't ask why, it's a long story). In those cases, USPS Registered is my preferred method.