Kickstarter only allows backers to pledge once to one specific reward tier only, BUT there are options:
1) You can create another Kickstarter account, which can use the same Amazon account/credit card information as your first account.
2) To customize your pledge to include multiple reward tiers (or multiple pledges to the same reward tier), Kickstarter also allows you to contact us directly and ask if you can increase your pledge to customize your own pledge. Use the "contact me" link under our creator name in the bio section (under the "Back This Project" section), and let us know in which unique combination of reward tiers you're interested. Be sure to ask us first before increasing your pledge, as (even if our homepage claims that there are rewards still available), five other email requests could reach us before yours. After we check the stock, we'll email you back and confirm if we can fulfill your request. If we can, all you need to do is simply increase your pledge amount and notify us when your new pledge has been approved. We'll then log the details of your customized pledge in our records, and if you've requested any reward(s) in limited supply, we'll immediately change the number of rewards available to reflect your updated pledge.
Don't see the answer to your question? Ask the project creator directly.