I agree with owlcatz, please visit your local post office and speak to the post master.
You can call them too, but it is a lot easier to deal with a human in person.

The mail does not go directly to the destination post office.  It is routed from your post office to a central sorting hub first.
Depending on how the envelope was packed, it most likely got caught in their automatic sorting machine.
Much like a copy machine, it has rollers to move envelopes around.
I received samples in the past that got gobbled up.  It look 3-4 weeks and arrived inside an another envelope.

If this is the case, not to worry, the post office has a people to deal with this and it will eventually make its way to the destination.