a rain check was originally given when an outdoor event had to be rescheduled because of the weather. If you went to the baseball game and it started to rain you got a ticket good when the game was scheduled again.
It has come to be used for events (such as sales) where the store might give you a ticket to buy the same merchandise at the reduced price after they ran out.
So 'NO rain check' means the low price will only be good as long as the in-stock merchandise lasts.