“Amazon vendor return – overstock”

I sell crochet grips on amazon through FBA. I recently received two full boxes of grips to my house, 500 in total. I did not receive any word before about these being shipped to me. That would be fine but I just looked at my account and realized they charged me FBA fees totaling over $900. I can’t afford to pay this, I don’t even make $900 / month in sales. Do I have any leg to stand on or did I screw something up? Has anyone else dealt with this?