1. It is NOT a scam. A lot of people have used cards here over a period of months and everyone is totaly fine. Do not worry about that!
2. Yes, you can use the paypal (though it is more expensive and you do not get the serial immediately). Look at the bebsite. It should tell yuo how to use it. But if you have a card, just use it! A lot easier for everyone. I repeat: IT IS NOT A SCAM!