On parking tickets, it use to be my rule of thumb to ignore them if in a different state but what many localities are doing is going to collection agencies. A different type of club since they cannot suspend your vehicle registration or deny it. Unfortunately cannot do it with the USPS.
Does not happen ~ over the years I've heard of a number of these guys getting belligerant and then finding themselves in a jail cell.
You enter the door you are in a federal jurisdiction. You drive on a road with postal delivery service, or federal funding, you're on a post road. Sure, they let you have your own state laws, but they can actually place their own requirements on those post roads ~ e.g. how far from the curb to put your mailbox, how high it's got to be, etc. It's subtle, but it exists ~ they also define 'gate' ~