Ethiad Cabin Baggage Fees - Photographer

I’m really frustrated with Etihad right now. I’m a photographer and I travel with a small camera bag that fits under the seat, but it’s heavy because it has gear and batteries. I tried to pay for extra cabin baggage and was told it is not possible. They only let you buy extra checked baggage.

My camera gear and batteries cannot be checked, so I asked what would happen if my cabin bag is overweight since I have no way to buy more allowance. They told me I would be charged at the airport for overweight cabin baggage.

I honestly do not understand how this makes sense. I tried to pay ahead of time and was told I cannot, but I will still be charged later for not paying. Has anyone dealt with this or found a workaround?

I’ve never had a problem on any other airline bringing one small bag that’s half the size of the standard cabin allowance, plus my camera bag that fits under the seat.

Any other photographers/media professionals dealt with the strict Ethiad rules? Again, I've even tried to pay for excess cabin baggage but they said I cannot.