If I got a letter regarding a unpaid fine for a DIU, must I pay it if its from an attorney and not the court?

I would contact both the court and the attorney that sent you the letter and inquire as to the fine.  Ask the court two things: one, if you can still pay the fine to the court and two, if the court hired anyone to collect on their behalf and explain why.  Ask the attorney by what authority he is collecting and for whom.  Do not agree to pay anything until you are sure that paying the attorney satisfies the debt.  Also, I bet the attorney has fees tacked on to the original fine so you need to check that out as well. Informed is the only way to make this decision. Good luck.