Can they do that legally? It said "today only" and "limit 10 per customer", not "while supplies last". There's no accompanying legalese, so shouldn't they be legally obligated to honor their offer until 11:59PM?
Edit: I'm certainly not a legal professional, so my opinion is not the law. On most print advertisements, I see advertisers give a disclaimer such as "We are not required to offer any mistakes in printing etc." This covers them legally since they need to advertise honestly.