I have written “USA” on all my cards so far in the last address line and I have had no problems so far with the delivery in the U.S. .
Mainly for the reasons of space on the postcard.
I work for the post office as a carrier…perfectly fine pieces of mail often show up with those stickers and we either deliver it or take the time to take the sticker off…but I am in a small town. I have more time than those in the cities. The problem is internal and when you KNOW the address is good you should be able to send the item right back out into the mail stream with the same postage that was originally used. It should not cost you any more because you paid for it to be delivered and a machine error, or human error was made. The post office sorting plants run faster in the usa than in any other country…and they actually run too fast and mess up a lot. Also, the plant employees take shortcuts sometimes and what should take 3 passes thru the sorting machines they will send on after only 2…which sends a lot of mail for close by cities to the wrong routes. The carriers find this mail in their delivery route pre-sorted mail and set it aside to send back to the sorting plant…a human is supposed to dump that mail into a certain bin to get it resorted, and we do believe they often just toss it into any bin, or a FORWARD bin…those letters will not be in the forwarding system and will get a NIXIE sticker on them. (NIXIE is the name of the system that handles all of the special exceptions and looks in the postal database to reprint addresses etc). The post office is understaffed in the USA all over, it has been an issue for many years and is one I don’t see getting fixed any time soon because of the way the post office is run.