Short answer: you can't pick multiple rewards as Kickstarter isn't a shopping cart per se. Here's what you do: 1) add up all that you want to back. 2) pledge that as a "custom pledge" 3) Then, when you get your backerkit survey which converts your pledge to a shopping cart, just select those items. You also can change your pledge at any time and select steel versus aluminum and color at that time. Make sense?