When your invoice becomes ``PAID`` for the first time, if there's a cart of inventory items attached to it, that cart becomes permanently reserved -- that is, all of the items within it are no longer available for other users to purchase. If an invoice becomes ``REFUNDED``, the items in the cart are released, which means that they are available for anyone to purchase again.

In general, although this means you *can* use negative payments to take an invoice into a *REFUNDED* state, it's still much more sensible to use the credit notes facility, as this makes sure that any leftover funds remain tracked in the system.