Fee for using a debit card?

[Kronos]

I had to use my debit card in my local shop this morning and was charged a 50p fee for doing so and wondered if this was legal?

 

[Kronos]

It was only 50p on a £5.60 purchase and did not really have an issue with it. But it occurred to me that this was the first time I had been charged and I use my card pretty frequently.