Originally Posted by
miikkak
In practice it's very complicated if one isn't BAEC member. Collecting the fare difference requires that they have enough information (address etc) to be able to send the bill which they often won't have if tickets are bought through OTA and paid through online banking.
If it is paid directly, the airline has details of the form of payment and should have addresses et al
If booked through an agent, it can send the agent an ADM and then leave it to the agent to address with the company/passenger that booked it