I hope I am understanding your question correctly. Why aren't you selling using a Fixed Price listing, and entering the quantity of 19? Make an ad that says "15 skeins of silk thread", enter the quantity available as 19, and if a buyer wants more, they can add that however many they want to their purchase. That is what I do when I am selling multiples of the exact same item. Selling with "lots" just confuses buyers. I always remember the KISS rule: keep it simple, s______. (i say that to myself, not you )