Advertising copy shouldn't open with, "Just kidding."
If that's the only way you can tie your headline to your main message, you need a better headline.
(One of the junior writers I work with just tried to bridge the gap between a clever but irrelevant headline and the main message with, "Just kidding." He won't be trying that again anytime soon.)
This reminds me of the old cliche ad with the headline that says simply, "Sex." The copy following it begins with, "Now that we have your attention..."
That idea sucked, too.