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

The agency says I reported earnings incorrectly, but I think I followed the instructions. I have read a lot of threads here and could not find my exact situation. I cannot afford to repay it all at once. Who should I talk to first?

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.