I picked up a valid pre-order invite from another party and used it to place an order for a PQ/3G. On the first screen I fille dout my billing and shipping address info then clicked the button to pay with AMEX. The website presented an error that the gateway could not be contacted and that I was required to contact NI Support to resolve the issue.
This was over 72 hours ago. I have since emailed support and preorders each day but only receive the automated response that they will respond as soon as possible. The NI status page just shows my order information with the status "Not Paid" and no other links to cancel or pay the order.
Some people have said that someone from NI will respond here sooner than via email but I see a number of others in the forum with similar payment difficulties and not many resolutions.