I just paid for mine (with CC and +3% - think I got it right ) and I can see how it happens. Since paypal automatically picks a payment source it's very easy to simply use the credit card you have attached to your paypal account and forget to add the 3%.

There's not a whole lot you can do I think. It's a combination of the payer doing things to quickly without reading and paypal making it too easy to do so.