Allow readers to select which books in a series are included in bundles.

Allowing readers to do this would mean that you could have a larger series, and have a discounted bundle for orders of two, but allow people to pick which two books in that series they would like, as some people may already have some, but not others, and they might not have them in order. This could be used for any size series, and a variety of sized bundles.

Meaning you could have a 2 book bundle for a 4 book series, and they could select any 2 of those books out of the 4 to be included in that discounted price.