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

The agency says I reported earnings incorrectly, but I think I followed the instructions. Any experience at all would help. I cannot afford to repay it all at once. 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.

Thanks for writing this out. I went through something close to it 8 months ago, and it settled down once I kept a simple log of every call and form.