A change that costs more is paid for before the order ships

When a customer's change to their order costs more, they pay the difference through Shopify's own payment link, and the order is held from fulfilment until they do. Once the payment clears, the hold comes off by itself. If they never pay within the window, Tacey undoes the change and the original order carries on, and a product that was added but never paid for is taken off the order and restocked before it is released. On orders that charge tax on top of the price, the confirm step shows the difference as "+ tax" so the customer knows the exact total comes before payment, and your records keep the exact amount Shopify calculated.