The PoS my wife's orchestra uses has a flat 1.75% transaction fee (I think there's an additional 10p flat fee for credit cards).
Anyway, it's not an argument about cash versus card, there are always costs for running a business – from cashing up and taking cash to the bank (indeed, if you're a small shop, there's always risk in having cash on the premises), stocking the shelves, throwing out the rubbish, etc. etc. In my experience, they mostly seem to be absorbing the charges.