I was logged in, and it instantly rejected my perfectly good credit card, sending me back to the cart contents. I resubmitted, thinking I'd entered a bad number, and it told me there was an identical transaction in the queue.
So I waited and checked my card - it was fine, no charges. I ended up resubmitting via PayPal. That worked (confirmed both by PayPal and an email from QMX), but after QMX processed the charge, the confirmation page said I'd paid with XPay - whatever the Hell that is.
Add it to it that the interface isn't standard and the graphics are broken and it's not exactly a confidence-inspiring experience.