You can add extras to every delivery or remove a one-time extra in your account, before the change deadline.
For one delivery: open your scheduled delivery under Deliveries and select the trash can next to the product to remove it.
For every delivery: go to Extras, set the quantity for each product and select Save at the bottom. To stop an extra, set its quantity back to 0.
Once payment has been requested (2 days before shipping), that order can no longer be changed.