PayPal, 2Checkout, Credit Card are three options that you might have to offer
Providing some choices is always good instead of just one..
Nobody knows what payment your potential customers will choose...
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Be sure to check out full service solutions as well which offer multi-currency support, order pages in multiple languages, EU and US tax management, and other international payment features. Full service solutions have major advantages over basic services like PayPal, 2CheckOut, etc.
Well, there are a few options here. The most common form of payment, I find is PayPal. PayPal is a secure payment-proccessing service that allows customers to pay with a click. If they don't have PayPal they can use a credit card via PayPal too.
You might also want to look into credit card proccessing accounts from providers like MerchantWare (for iPhone), though I don't have any experience for MerchantWare.