If you have a package stolen from your mailbox then USPS will not reimburse you for the package.
That is because it was the thief who stole the packages fault and not the USPS.
USPS will only reimburse you for packages that are lost or damaged and not for stolen packages.
If you do have your package stolen though you need to report it to the police and it's a good idea to have a camera recording your mailbox so that you can catch the person who stole the mail or packages.
However if the package was stolen then USPS will usually do nothing and you're usually out of luck unless you can catch the package thief.
The stolen package also should be reported to the USPS though and the USPS postal inspector who is the police for the USPS.