You can add-on extra items like the Emerald deck or coins by clicking the 'Mange Your Pledge' button at the top right of the project page. After you click it, you will click 'Change Your Pledge' where you can then update the total 'Pledge Amount' to include the additional cost of your extras. From there just continue to confirm! You're all set!
NOTE: You do not need to actually change your tier. All you are editing is the numerical value. Things like coins may not be listed in tiers. They are just added to the current tier that you have.