I was trying to buy some Buckyballs for my niece for Christmas while in her presence, and using her laptop. I tried to check out with a credit card. Since your web store does its payment transactions through Paypal, and since the credit card that I wanted to use is already associated with my Paypal account, I could not use that card. I then tried to use my Paypal account, but managed to get into a perpetual loop that kept returning me to the same order confirmation page. After two or three iterations of the same loop I finally gave up and handed her cash; she'll probably try the same purchase herself today using a parent's credit card.
I am an IT executive with over 30 years in systems infrastructure and software engineering experience, with almost years in E-commerce, and an still doing hands-on network engineering, system administration, and software integrations. I wouldn't call myself an unsophisticated user, though in this case perhaps an impatient one. If a process is too complicated or inconvenient for me to cope with, then your average customer doesn't stand a chance.
I doubt that my scenario (having a Paypal account, trying to use a credit card associated with that account, then trying to use the Paypal checkout) comes up too often, but I expect that when it does it results in a lost sale. You might want your QA/QE team to try to reproduce the results.