Select the largest reward that you want (or keep the one that you have currently selected), then add the dollar amount for the additional rewards that you want to receive to your total pledge.
Then, send us a personal message from the Kickstarter page (not a comment!) and let us know what additional rewards you want. Just make sure you've added the correct dollar value for the additional rewards. We will keep track on our end what additional rewards you want once you've let us know.
If you are international, you DO NOT have to pay shipping twice. Just add the dollar value for the additional reward you want on top of your first selection with shipping.