Balham
S24SU Seer
You pay a £1 booking fee every time you purchase a ticket by card. I highly doubt the box office pay the card companies £1 to cover the cost of the transaction.
As far as I know, the EFT providers charge between 0.5% and 3% to process a card transaction, with the actual fee dependent on card type (Amex being far more expensive) and size of the business.
I doubt United will be paying more than 1.5% but they do have to post the tickets out too, which I think is included in that £1?
I might be wrong, mind!