Got an overpayment notice, what now? after 11 years as an electrician's helper

I received a letter saying I was overpaid about $550 on my claim. There are 11 people counting on me, so I am trying to be careful. I did not notice any errors at the time. Has anyone disputed one of these?

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.