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

I received a letter saying I was overpaid about $350 on my claim. This is my first time dealing with any of this. I did not notice any errors at the time.

Collect your pay stubs and certification records for the weeks they name. Mine showed the error was a date mix-up.

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.