Purchasing RP using mastercard etc.

I wanted to purchase Rp on league but for reason it my league will instantly quit when I have to put in the expire date. I have had this issue for about a half year now and have to consistently ask my friends to buy for me simply because of this bug. My friend has also started experiencing this issue. Maybe this is only on a bug on the MAC , but if anyone wonders. I use a macbook pro retina inch 13. Please fix this riot so I can buy skins etc , and help your business grow :) . IS ANYONE ELSE EXPERIENCING THIS ?? lets get this fixed!