The email header says sold, the email text says listed, if you go to the offer acceptance page, it says "...items listed and sold for $100 or more", so I think if it's an auction the start price has to be at least $100 to qualify for the discount. 

At least for GTC listings created during the promotion, the discount applies if it sells before it ticks over, so you get a 30-day chance of a discount....if you have and sell stuff for $100+, which I don't. :^O