Selling Extras in a Pro Site Design

Hi all,
I have a new service package I want to sell via the Pro Site's plugin to sell sub-sites.
I have a number of packages that will form the basis of the price / packaging but there will be certain extra features that will be optional. How do I set that up? I'm happy to manually enable the extra features but want to work out how to then have that extra package included in the client's monthly fee.

For example, three Pro Site layers:

Optional banana = $20/m
Optional 2 bananas = $25/m
Optional orange = $35/m