[OFF] Double charges to credit cards

This question is not necessarily PHP-specific, though we are running PHP classes. Online store, credit card authorized at time of order, credit card charged at time of shipment (could be anywhere from a few minutes to a couple weeks later). Standard stuff. But customers are complaining that they're being double-charged. Turns out that both the authorization and the charge are showing up on their statements. Our merchant provider, ConcordEFSnet whose API we're using, says that this is just the way it works and there's nothing we can do about it. Sounds too incredible to be true. I've never seen anything like this on my own credit card statements, and I can't imagine that Amazon, et. al. have this problem, though they follow our same business process. Can anyone shed any light on this? - Brian