Got an overpayment notice, what now? - any advice?

I received a letter saying I was overpaid about $300 on my claim. Any experience at all would help. I did not notice any errors at the time. Who should I talk to first?

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.

If you are unsure, free legal aid in your area may help for this kind of notice.