I don't think there is a fee if you haven't used your 20 allotted freebies in that category for the month. But it does use up one of your alloted slot. If you don't notice it like I didn't yesterday it took two of my 20 freebies in that category. After that if I pass the 20 in collectibles and 20 in others ebay starts charging us 30 cents per listing. If they sell, they refund the 30 cents. If they don't it can add up real quick.
Better off using the 30 days which is what I wanted to do. That way you can decide if you want to relist or not.