I've got a product that is a gift subscription to a magazine. The product page allows a user to enter the gift recipient's address. I want to write an observer that, upon clicking 'Add to Cart', will intercept the action and do address validation with the USPS. If it fails validation, I want to redirect them back to the product page with all of the form data still intact. Is there an easy way to do this using some super nifty ZenCart function?

Kevin