You fell down the mechanic trap at an oil change place and they charged YOU for it?? Honestly, if I was you I would call the better business bureau or something, because that is no way to run a business. They should have had safety signs, at the least, and a more watchful staff. To me it seems like they would be more worried about being sued than charging you extra money.
Maybe I am missing something from the story?
Either way, that sucks your mother is mad at you. I can relate to having parents that would get mad at me over things that weren't my fault. Hope all is well, if you need to talk to someone feel free to send me a message : )