This is what CC disputes are for. You try once to resolve it. If it doesn't work, dispute that part of the charge which is incorrect and go about your business.

Your allowance is stated on your e-ticket receipt (per DOT rules). That is what the agent saw. Presuming that your ticket was purchase after 4/8, you were permitted 2 bags. Presuming that your bag weighed more than 50 and less than 70 pounds, the overweight charge was $100.

So, file a dispute for the $150 overcharge. Keep it down to 2-3 declarative sentences and refer to the e-ticket receipt.