Bundles in preorder mode are generally cheaper (around 50%-75% of the price) than when launched, and would grant you all tiers and products that are present at time of launch. In exchange for the discount, you will not know what exact products are in the bundle, only the number and types of products available. But don't worry, if you watch chat, people will try to guess what items are in the bundle, and occasionally staff will drop hints on bundle contents.
Preorders are only available before a bundle has launched, and are not present on every bundle. Tiers added after the bundle has launched may not be added to preorder purchases.