Changing an order's items — when it's allowed

You can change an order's items (quantity, add, remove) only while the money hasn't arrived yet — the order is waiting on payment, the method is cash/bank transfer/cash-on-delivery (never card), and courier prep hasn't started. A paid or card-settled order can no longer be changed — a card payment is already tied to the bank and can't be rewritten, so in that case cancel it and record it again. An edit also adjusts stock: raising the quantity reserves the extra units, lowering or removing a line gives the reservation back.