I called (I know, I know - calling often ruins a deal, but with this 'deal', not calling will end up with only getting 50 and therefore negating the whole 'deal')
They said it was a mistake on their website and offered me either:
1) A 20% discount toward ordering a real 500 case
2) A refund of $$3.99 (=50 free covers).
Thanks Daniel for the 50 free covers!