AFnord: I just type in my credit card number each time I do a payment through PayPal. I know it by heart anyway, so payment is swift and painless that way.
MorphysLaw: Isn't that a bit double-upping? I mean, most places that have the option of paying through Paypal also have the option of paying with a credit card. Are there any benefits to paying through Paypal with a credit card? (Just curious, I don't have a credit card myself. I'd rather not end up in debt.)
Like AFnord says, most of these sites use a payment processing service of some kind, but they work just as well with a debit card as a credit card. You don't have to end up in debt. Mostly, US and UK residents talk about credit cards as a catchall because they're credit based societies. If you're going to rent a house/car, get a contract phone, etc., your credit rating will be checked to make sure you're a trustworthy individual. No credit history in these countries can make life complicated.