On Cellar a shopper sees one cart and pays once. A winery sees only its own bottles, its own delivery fee and its own delivery slot. The Split-Cart Kraken lives in the gap between those two views, and it has an arm for every seller.
The trick is to make the split a fact of the data and not a trick of the screen. A cart is a list of lines, each line belongs to a seller, and the app groups by seller only to show it: a header with the winery's rating, its fee and its slot, then its bottles. Checkout turns one payment into one order per seller in a single transaction, so either every winery gets its order or none does.
Money is where it bites. Fees belong to sellers, so the total is a sum of subtotals plus fees and not a percentage of a basket, and every one of those numbers is computed on the server and shown on the phone, never recalculated there. Keeping each seller's order moving afterwards is a story for another entry.