On September 24th BA announced it would be charging people for advanced seat selection. When the scheme was launched yesterday you could only reserve your place offline. 
The reservation service will remain an offline proposition until December when passengers will be able to pay online for their seat reservation at the same time that they book their flights. You will be able to modify your booking under the “manage my booking” section after the ticket has been purchased.
Until the online seat reservation is available you will have to pay by credit card by phone. A second transaction to you seat booking will be recorded once this has been done.
In a recent poll carried out by Businesstraveller.com 853 people were asked if they would pay for their seats. Out of those who responded only 19.6% said they would be willing to pay to book their seats.