PayPal is very simpel to use. Another suggestion is Stripe (I have no affiliation with them but our team just got an invite to try out their product and it is great), ask for an invite, their solution is very simple to use both for you and for the users - https://stripe.com/
I'm always happy to use Pay Pal. At checkout I can just sign in to my Pay Pal account, click one or two ok buttons and that's it. No entering of addresses or credit card details, no creating another user account, etc.