Dang, I thought Shift was an entirely different company. So it sounds like it's a little like using your credit card at a gas pump, they hit the card to pre approve enough to fill the tank. In this case they pre approve enough for tip etc.it sounds like. Ok, headed back up to your link for Shift.
They charge a little bit extra and then refund when the BTC goes through. I think it was less than a dollar but when I look at my shift wallet I couldn't see any extra costs.