Dang that sucks that they're trying to charge you to change the order. Can you just cancel the order all together and then submit a new order? I've always had great customer service from them so this is kind of puzzling.
I once started an order, then changed my mind and didn't complete the online order form. I guess I got far enough into the process where it submits the order into their computer system, but I never paid for it. A few weeks later they emailed me asking me if I still was interested. I was but was going to wait till my next paycheck to order one. They were prompt and friendly in all their emails.
I found this on their site:
http://www.comp-tac.com/faq.php#9
Can I change or cancel my order?
Orders in the “Pending” status can be cancelled or changed at any time. Orders in the status of “In Production” will be charged a $20.00 change or cancellation fee to cover material and handling expenses. Please ensure your order is correct before you submit your order and contact us immediately if there are any changes.
Hopefully your holster isn't "in production" yet...