Kickstarter only allows you to pledge at one reward tier.
To add additional rewards when you pledge, simply add the costs of the additional items to the amount of the tier you have chosen and put the total amount in the box where it says "Enter your pledge amount".
To add additional items to your existing pledge click the blue "Manage Your Pledge" button located directly under the project's goal on the right side of the screen. Add the costs of the additional items and adjust the "Pledge amount" to reflect the items you've added.
When the project reaches it's goal and closes every backer will receive a survey. The survey asks questions about your reward tier and you are given the opportunity to list the rewards you expect to receive in exchange for your pledge.
International shipping isn't automatically added. Please add the international shipping to your base pledge amount if you live outside of the US.
It's kinda cumbersome but it works. If you're still not sure what to do or if you have any questions don't hesitate to ask. We'll be happy to help.