I haven’t been in Carrefour for a while , so I am not sure about it now, but some of them sometimes doesnt accept Mastercards ( ours , monzo are all mastercards)
Unless UK , Many small and many greedy banks always make you pay a fee if you do not use their own debit/credit cards , is not Starling / Mastercard issue here.
Is just how Spanish ATM works. (Always check different Bank if they are making you pay fee , and if
are asked to charge in Euros or Pounds , always select Euros. ( all Spanish Bank exchange rate are pretty bad)
Usually Caixabank (Catalan Bank ) and BBVA (Basque Bank )are the ones less likely to incur on fees while taking cash from their ATM’s.
Regards and enjoy Spain