Where cart orders show up for you
Orders placed through the cart land in the same place as your direct "Buy for" sales — your commerce orders list. A cart order that included several products comes through as one order for the shopper, paid in one charge to your account, so it's easy to fulfil and reconcile.
Refunds work the same as any order: open the order and issue a full or partial refund. Because each business's items settle separately, you refund only your own items — never another business's share of a shared cart.