The headline prices on BA.com do not include these extra fees and they are only discoverable after you enter the passenger name and proceed to the payment page so we summarise the fees here:

Seat Reservation Fees from £11 on short-haul flights e.g. London-Zurich

If you pay using a credit card or PayPal account billed in the UK, you will be charged a fee of 1% of the cost of your booking, up to a maximum of £20.00 per person.

BA does not charge this fee on the cost of any seat reservations, travel insurance, or donations. The fee will not be refunded to you if you cancel your booking.

There is no fee when paying with a debit card.