Travel Agents charge what they like and what they think they can get away with. Any travel agent can cancel a BA ticket within 24 hours and they will be charged nothing by the airline. However, they can and will charge a fee if they can get away with it.
As I have said before TA’s will often say there is fare difference to pay when making dates changes which can be true but is often NOT the case. I have caught my TA out on many occasions!