The buses seem quite straightforward here: http://www.krakowairport.pl/en/61/176/36/en As I understand it, you want the 3,40 ticket (unless you're eligible for reduced rate, probably over 65, possibly over 60).

You can also buy the tickets at the RELAY shop or at the bus driver in case you don't quite know how to work the machine. I'd be very surprised if they sold you the wrong ticket if you asked either the driver or at RELAY. I would think they accept credit cards at RELAY, although I would not normally buy *only* a 4 PLN ticket with a credit card in Poland.