Use Stripe - there is a small processing fee but it's extremely simple to use and free to set up. I have built many-a-website recently with this payment system. I personally prefer it over PayPal. However, if you go the PayPal route, use Payments Pro (there is a first-time charge and then transaction/processing fees).
Edit - I just read the recent comment from OP about being in Italy. Stripe is not available in Italy yet, but looking at all the countries that are in BETA, I'd be willing to assume that Italy isn't far behind. With that said, you are better off using a brand of PayPal or Google Merchant since they have much better international support. However, sign up for notifications on Stripe in Italy. You'll hate all other processing systems once you make your first transaction with Stripe. :)