With a debit or credit card transaction there is a capped charge to the vendor. (This charge is usually hidden, incorporated into the price; the only exception brought to mind is DVLC who will bill you the road fund licence and then add the debit card charge, or the more expensive credit card charge, separately - they they are not, theoretically, out to make a profit.)
There is another charge added on top if the payment is facilitated by PayPal or some other transactional agent, plus additional accountancy costs (small, but a thousand small transactions . . . ).
Some vendors will include any agency fees in the price, taking benefit from those who pay directly.
Somebody has to pay somewhere, so it all comes down to the vendor's pricing policy.