Once you have selected a pledge level and backed the kickstarter, you will see an option to 'manage your pledge' on the right of the page below the 'you're a backer' message. You have an option to increase your pledge amount, simply add together the cost of the extra miniatures you want and then increase your pledge by that amount. there is no extra shipping cost. We will email all backers once the kickstarter finishes to check which extra miniatures you want. There will also be a chance one the kickstarter has ended to add extra miniatures at the kickstarter exclusive prices before your pledge ships.
Don't see the answer to your question? Ask the project creator directly.