Your first mistake was paying the debt when you had no obligation for the debt. How were you forced to pay?
When you are an authorized user on an account, you are clearly identified as such under the ECOA code submiitted by the creditor. Being an authorized user does entitle you to obtain benefit of credit reporting of the account to your credit file, but clearly does not establish any legal obligation for debts under the account
In my opinion, the creditor necessarily knew this, so any action they took to collect the debt from you would be illegal. When assigning the debt to a debt collector, they obviously provided your name as a responsible party for the debt, which again was improper. Thus, any communication to you from a debt collector would be an improper action on the part of the debt collector, and a violation of FDCPA 605(b) as a communication with a third party relating to a debt they did not owe.
I would immediately send a direct dispute to the debt collector (I would not dispute through the CRA, or get them involved), providing your evidence of non-ownership of the debt, and dispute the accuracy of their reporting of a collection against you to your credit report. It should be deleted in its entirety, regardless of the fact that you paid it. It was not a legitmate collection.
You might also want to consult an attorney regarding their FDCPA violations.
And, yes, of course, get your authorized user status removed from that account. Contact the other party and have them contact the OC. Or contact the OC directly and request removal.
Starting Score: 675