Draw focus to checkout errors

[jasonhendee]

I know this has been talked about on the board in the past, but when we started having customers tell us "I clicked the pay button, and nothing happened..." I decided to use a ModalPopupExtender to make these kinds of errors unavoidable. It seems to be working great! If you're not sure how to do this, check out the already-present terms and conditions extender on the same page to see how it works.