Also, consider SEO if this is for the web, but user didn't specify. Maybe include the primary keyword in the headline.
Offer valid while spots last. No credit card required. Terms apply. xnxn free exclusive
I should also consider potential legal issues if "xnxn" is a registered name or if there's any trademark concerns. But since I don't have that info, maybe just keep the text general. Also, consider SEO if this is for the