Got an overpayment notice, what now? (update)

The agency says I reported earnings incorrectly, but I think I followed the instructions. I do not usually post, so bear with me. I cannot afford to repay it all at once. Are payment plans possible?

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.

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