Are you looking on the Seller Hub?
There is a specific link under Marketing for Promotions.
And on the Sell Your Item form we can add Promoted Listings, and Best Offer parameters, as well as a shipping cost for "each additional" item on purchase.
I only use Promoted Listings since it fits my personal "something for nothing" preference, in that I only pay for successfully promoted listings, and no fees are charged on items that are Promoted but do not sell.