Son almost misses flight because of mystery ticket fee

My son was traveling home for Christmas on a ticket that I had purchased in advance for him. He was given a boarding pass, checked his bag and proceeded through security to the departure gate.  Once at the gate, he was refused boarding because there was a 'fee due on the fare'.  No other information was given about the fee and he was told that they couldn't check on the details or verify anything - all they could do is take his money and let him board the flight.  

Being a broke college student, he did not have the ~$85 ransom payment in order for him to board.  He called me with about 20 minutes until the departure time in a panic because he was going to miss his flight.  I called the JetBlue reservation desk and after several minutes verified that there was no amounts due or any outstanding fees.  The agent on the phone was adamant that had there been any fee due, the ticket would not have been issued at the time of purchase.  By the time I was able to get off the phone with the agent and attempt to contact my son, he had already boarded his flight and had turned off his phone.

From my point of view, this was nothing other than an attempt to collect money from a passenger at the very last minute. "Pay up or you don't get on the plane."  Had he (or possibly countless other passengers) been able to pay the ransom fee at the gate, he would likely have done so simply to be able to board the plane and dealt with the issue later.

I would ask that anyone else who has experienced similar 'hostage' scenarios, please put your comments in writing.  You pay your fare and there should not be any additional fees requested.  Extortion, ransom, whatever you want to call it. Bottom line is it's wrong and needs to be addressed.