Add-on's are purchased on a per listing basis.
Spotlight Listing: Display your listing on the homepage's spotlight carousal for everyone to see.
Premium Listing: Display your listing on all sub-categories of the main category your listing is listed. Ex if your selling an electrical outlet your listing would be under the main category Electrical. This means your premium add-on would show your listing under all the sub-categories within Electrical.
Featured Listing: Display your listing at the top of the sub-category your item is listed in.