Nothing, they do nothing 99% of the time.
On occasion they open them and if it contains medicines they refer it to the FDA who then sends you a letter. Sometimes they open them and send you your drugs anyway, it'll just have green "INSPECTED BY US CUSTOMS" tape on the package. You have to imagine the volume of packages that are coming into the country through a very few points of entry, especially small parcels. They just don't care that much and have bigger fish to fry.
Personally I like Registered Mail that requires a signature, I've never had one get gotten as it requires extra work and collaboration with a USPS postmaster even for customs to open them.
Its not like the airport where everything goes through the Xray machine and a guy is looking at all of it.