In my experience with a nonprofit, on whose board I serve, this is exactly the case with PayPal. Even if you're very upfront before you send people to PayPal that yes, they can pay with a credit card, once they arrive at PayPal they get confused. I've even had PayPal show me a popup ad for a PayPal branded Mastercard when I was trying to checkout somewhere. Their UI isn't "branded" like the site that the customer was just on so that's confusing in and of itself. In addition, paying with credit card on a PayPal checkout page isn't the main action - you have to search for it. Combine all of those elements and you get a much lower conversion rate.