You can see the amount of credits in the upper right corner - it's the sum you've already paid through Kickstarter, and it's appearing as the store credit to be used in Gamefound Pledge Manager to get your pledge.
Please use the credits to choose the pledge level you've went on with on Kickstarter. You're also able to upgrade the pledge level and some addons and/or expansions over the limit of the Pledge from Kickstarter. Simply, you'd be able to pay the difference in the final checkout. As in the regular webstore, in the checkout you'll be able to provide the shipment info and pay the shipment.