The quick answer is that, if it isn't really, really, obvious, that a company is just lying or acting fraudulently, they'd beat any suit. They are giving their customers the $5, are they not? (In their interpretation, anyway.)
You might try complaining to their corporate office, and have a few other customers complain. You might also call the BBB and see what they think.
On a lighter note, while you are at it, complain about their "10 Items or Less" signs that they have in their stores; it should clearly be "10 Items or Fewer".