Red Flush is a Microgaming Casino. So if you were an existing player before Microgaming blocked new U.S. players then you are good to go.
It's marked as "non-u.s." on the main page because they no longer accept new U.S. players.
Are you sure that the piece of mail you received was from Red Flush Casino? It kind of sounds like you might have mistaken it with another casino (perhaps with a similar name).
If it is indeed Red Flush Casino, then I would just make sure you read all of the terms on the piece of mail that you received and you shouldn't have any problems.
And as always, if you do run into any issues, just let me know and I'll be happy to help out.