No, because the postal service doesn't have access to read the contents of letters and parcels in transit. Not only do they not have access, but it is illegal for them to access the contents without a warrant.

This is why common carrier status is so important. Snap can't possibly police it's user base.

Which is probably why they're involved in investigations and prosecutions of same.

However, the postal service doesn't retain a copy of the data it receives, nor does it even open or view said data.

Nah, they don't open the letter, make a copy, store it, etc...