When Jason is asking the mailman at the local post office to mail his letter, the mailman responds by referring to the Postal Service. President Nixon semi-privatized the mail service in 1971 to the US Postal Service. Prior to then, the mail service was a fully government-funded body known as the US Post Office Department. This is what that mailman worked for, and to which he should have referred.
As Grandpa, Jason and Verdi enter the estate's gate you can see what looks like production trailers on the left of the screen and again past some trees.