I'm not sure what your question for other members is.

You are not exactly a newbie, so presumably across eBay policies.

Including, but not limited to, timeframes.

A year is way out of time for you to get eBay to do anything. If they 'undercharged' your buyer, it would usually be a fault with YOUR listing and how you have set variations up.

Either way, you are way out of time for any action/remedy through eBay.