Optional items during checkout

Hello-

I am looking to make something happen during checkout that I can't quite figure out. My client has items for sale that are parts of bundles. Once a user buys the main bundle, they can then have the opportunity to purchase additional components of the bundle at a discounted rate. For instance, if a bundle consists of 5 core items, during the checkout process they are then allowed to buy additional copies of items 2-5 at a discounted rate. How I would see this happening is during the checkout process, once they have selected the main bundle, they then have a screen where they are given the option to buy additional copies of these components. For instance, I add the main bundle to my cart, on the next screen during checkout I could then (and only then) be given the option to purchase these additional items in a subsequent screen. The reason for this is because we want to prevent users from buying the components a la carte while avoiding the main purchase price.

How would I best go about that?

Any help would be greatly appreciated!