A service for whom? It has already been paid for by the users when they bought the iPhone.
Companies that prefer or only take cash tend to do it for tax evasion rather than cost purposes.
U.K. fees tend to be about 1-1.5% all in, for cash or card (excluding cost of employee taking cash to bank). Things like pix, net, grocers about 0.5-1%. Nets QR is cheaper than Nets rfid.