You can give credit to a customer, say for a damaged or missing item or if the customer buys a gift card.
Go to Pickups, search for the customer, and click Credit. You can change the dollar amount by clicking the calculator icon. Then select "No Charge" or a payment method if the customer is paying for the credit.
In the Pickups page click the "Apply Credit: $X" button in the bottom right to apply the credit to the order.