Add Upgrades to Product Versions

The versions function is good, but there should be an option to allow product upgrades like on Gumroad. Sellers can already add versions that can have different or more items than another. There should be a way for sellers to set up a base product and versions that come with more items at a higher cost. Then customers should be able to buy the more expensive options at their own will like upgrades that will take their current version into account so they only pay the difference.