Contact Frontier. See what they say/do. Those gate agents are all contract employees through staffing agencies. They can also track how many people were overcharged, etc. through their system. Not that any corporation would ever admit guilt...

What is weird is when I buy my tickets at the airport, the seat is automatically assigned right when I book... (no fee)...

So I guess it is different if you buy via the website? The seat assignment is manual?

Not sure about how the recording of the gate agent helped at all other than to probably escalate the situation... the "charge" would have spoken for itself regarding the agent's activity and conduct.