Your example of the payment button is exactly how Dutch-only iDeal works and has adapted well to mobile. The pop-out however is to your own bank's app/website,

Your example of the payment button is exactly how Dutch-only iDeal works and has adapted well to mobile. The pop-out however is to your own bank's app/website, which works cause there are just 10 banks. Transaction fees? < 0.50 € per transaction