It's really simple to top up. Just go to "manage your pledge" and increase your pledge by the appropriate amount. You can top up as many times as you want with as many items as you want!
To give an example, if you're pledging $25 and you want to add a T-shirt then you select the $25 reward and add $30 to your original pledge. Now your total pledge amount equals $55 ($25 + $30 = $55). So it's pledge/reward level + top-ups = total.
Once the Kickstarter term ends, we will send out a survey to find out exactly which top-ups you want in addition to any other information we might need to send the rewards to you.
Please feel free to contact us if you need any help with this.
Silver Member ($50) + USB Goat ($50) = $100 pledge
Gold Member ($100) + USB Goat ($50) = $150 pledge
Gold Member ($100) + 2 additional T-shirts ($60) = $160 pledge