Double charged on a subscription for 2 years - Apple only refunding 7 months

2 years ago, I switched an app from a monthly subscription to an annual subscription. Apple charged me for the annual subscription and continued to charge me monthly subscription fees. I just noticed today and called Billing Support - they were very nice, but told me they could only refund 7 months, and I should be grateful since this exceeds the terms and conditions of the App Store.

The service rep claimed I should have noticed (a fair point - I definitely should have noticed), however, that shouldn't mean they aren't responsible for what they overcharged me. Basically, I was over-charged $326, and Apple is refunding $91 and telling me that I should feel lucky I am getting anything refunded.

I'm pretty irritated. Any suggestions for getting the full refund?