I have bought from them a couple of times - most recently last week. They put a hold on your credit card for a small amount (under $3 I believe) if they believe it may be a fraudulent charge and will send you an email about it. All you have to do is check your online banking to see that charge (assuming your online banking shows charges that are on hold but not processed to your credit card). Once you have the amount, you click the link in the email from
Undercover Tourist and put in the amount on the page it takes you too and that verifies your purchase. That amount is released and the full purchase goes through. Like I said, I've had this happen twice and no issues at all. Actually kind glad they are verifying it to ensure it isn't fraudulent. They were amazing with their customer service last time I bought tickets through them and phone in a panic when my mother in law forgot to pack them and got to the airport without them and there was no way to get the tickets before the flight left.
(Both times I've used my USD BMO Mastercard.)