In message , Richard J.
writes

I could perhaps understand him charging a small fee for processing a
credit/debit card transaction (though it's time that nonsense ceased),

Credit card payments typically cost the retailer 2.5% of the transaction
value - cheaper deals are possible (largely for big retailers) but some
systems cost much more. In addition, the retailer has to pay a monthly
hire charge for the terminal, an initial set-up fee and phone charges
for each electronic transaction.

I don't know what commission TfL offer, but it would need to more than
cover the above sums - just the Credit-card charge alone on £40 could be
more than a pound, so an 80p surcharge by a small retailer might be
considered generous.

However, debit card transactions are more reasonable - typically around
20p plus a proportion of the above expenses. In that case, an 80p
surcharge is over the top and suggests to me that the retailer hasn't
understood the big difference between credit and debit cards.
--
Paul Terry