The flat fee on every sale
The gap between what something cost you and the price you stop losing money at is wider than most people expect, especially on cheap items: Whatnot takes 30 cents on every single sale whatever it sold for, so a $1 item pays a far bigger share in fees than a $250 one. These floors are an estimate after Whatnot's cut, not a promise, and they are a fact about your stock rather than advice about where to open.
About these numbers. What you paid comes from what you actually paid. Fees come from Whatnot's published rates, including the flat fee charged on every single sale -- which is why selling a case whole and selling it pack by pack give different answers.
Whole against broken up
Most of that difference is not the 30c per sale -- it is that Whatnot takes its cut of the postage on every parcel. Selling it whole is one parcel instead of many.