They are happy to take anyone's money! You can just take a cab over to Atlantis; tell the driver you want t o go to the casino. Unless you are interested in a tour, I wouldn't take the excursion.
We have booked at CS so we can use the pools and slides, which are not included in the tour. If you are just interested in the Digs, you can pay right at Atlantis. I heard that the price has gone up to $29 for the tour; pool not included.