We can often refund customers for the difference in price between the shipping methods. If the mistake results in a delay that make the product no longer acceptable to the customer, we'll give customer full refund and pay for shipping the items back to us. Alternatively, customer can refuse the package at the time of delivery. This happens very rarely. Our on-line charge system does not allow charging the same credit card more than one time for the same order and leaves a good record of charging history.
Manual charge is more susceptible to human error.
Such situation is not a double charge. Online credit card authorization automatically allocates the funds for the transaction, but that does not mean that the credit card has not charged or the fund has been sent. We do not know how to get the hold on funds released. It is also possible that one of the charges have been credited, but the credit shows up on a later date. Once informed, we'll launch an immediate investigation to confirm the double charge.