Weight class
The shipping weight tier a seller assigns to an item, which determines what the buyer pays for shipping.
On Whatnot, sellers assign each item a weight class, and that tier determines the shipping cost the buyer sees at checkout. A single trading card, a one-pound box of comics, and a pair of boots sit in different classes and price differently to ship.
Getting weight classes right matters in both directions. Set them too heavy and your items look expensive next to identical items on another seller's show. Set them too light and the difference between what the buyer paid and what the postage actually costs can land on you. Weigh your common item types once, set honest classes, and your shipping economics stay boring in the best way.