Has anyone recently purchased tickets on this website. In september we want to take a night train front krackow to vienna and want a couchette.
By "this website", do mean Rick Steves which only links to Rail Europe?
This site is agency for Polish rail tickets: http://www.polrail.com/en/st/tickets-reservations
It appears that since it is an international journey, you will have to pick the physical tickets up in Krackow. It looks like it is a little early to book September tickets. Price seems to be $110 equivalent (price is actually in Polish Zloty) per person in a 4 bunk couchette.
You can see the Polish rail system service at http://www.polrail.com/en
The website allows customers to reserve and buy tickets, but it does not have an e-mail printout service. Tickets can be sent by courier or regular mail (the site suggests having them sent to your hotel.) Or you can pick them up in person when you reach Krakow.
www.seat61.com discusses the service: https://www.seat61.com/international-trains/trains-from-Warsaw.htm#Warsaw_to_Vienna
Scroll down about half way.