Kickstarter only allows backers to select a single pledge level.
To get an add-on to your pledge simply add the cost of all the add-ons you wish to your total pledge amount.
For example, if you want to add 1 Sewer expansion ($50), 1 Oblivion Starter Set (50$) and 1 Accessory Set#1 ($9) onto an existing 2 Oblivion Starter Set pledge level ($105), you just add these together ($50 + $50 + $15 + $105 = $220).
Then, make sure you’re signed into Kickstarter. On top of the project page, just under the days to go, you should see "You pledged $..." and next to it the blue button "Manage".
Click on it and in the new page click on "CHANGE YOUR PLEDGE" in the lower right side of your screen.
In the new page select your reward and enter the new amount you want pledge (in the example $220). You don’t need to touch your pledge level selection. Once you are sure that the amount is correct, press CONTINUE. That’s all you need to do for now.
After the Kickstarter campaign closes, you will receive an e-mail from our Pledge Manager. At that time, you confirm all your specific set selections (and your shipping address).
At the end of this process you will receive a confirmation of your final order.