You can add items to an existing order up until the cutoff time. For delivery orders, the cutoff time is when your order has been assigned to a driver and sent out for delivery.
Simply add items to your bag and proceed to checkout. In the "Delivery Date" section of checkout, you will see the option to "Add to existing order." This will submit your new items to be combined with your existing order for the original pickup or delivery time.
If you need to remove an item from an existing order, please cancel your order and place a new one.