Doctor bill is going to be sent to collections what do I do?

While seeing my doctor I switched jobs and forgot to switch my insurance. So they sent me a bill for $400. Well I didn’t know this because I do not get all my mail because it’s redirected because I moved. So I happen to go to the post office and they gave me mail from months ago. Luckily there was one in there from collections saying I have until Dec 15 to contact or I guess it’ll be sent to collections. I don’t mind paying on it like $50 a week. But is it too late? Who do I contact? I also saw online is a medical bill is under $500 it won’t hit your credit if sent to collections. I need help here.. I’m 25 and never dealt with this before lol