If you want to increase your payment after you have purchased a bundle, you can simply do so on your purchases page. Go to My Purchases and find the active bundle that you want to increase your payment for. Click on the blue Donate More button, enter the amount you want to increase your payment by, pick a payment method, and click Increase to add more to your current bundle payment.
If your profile says “Pay $___ more and get the following products” then you can increase payment to the designated amount and qualify for the next tier of products.
For Build-A-Bundles, you cannot directly add more products through this interface. However, you can ask support to add the items after you increase your payment, so you don't need to buy an entire new bundle.
You can only increase payment when the bundle is still available for purchase. For limited tiers, they will be added only if the tier is still available.