I appreciate that it might seem a lot on a small transaction, but as I understand it debit cards have a fixed fee transaction amount. 50p could well be the right range if the volume is small. I mean they could well be charged in that region by the bank.
Having said that, I too never like to see additional charges added like that.
I suppose on many small items there is only a very small profit margin so a fixed fee could really be a problem there.