Amazon never charges for pre-orders until the item is shipping, because their price may change before release day. If you order today, and the release-day price is $55, you'll get charged the $55 on the day.
Thanks. I figured that was the case but just wanted to be sure.
But if it ships at $55 and the price goes down again (not likely) while your copy is in transit, you're hosed.

Amazon has a pre-order price guarantee, but that ends once the item is prepped for shipping. It's not like a retailer like Best Buy which will give you a refund if the price goes down within 30 days of purchase.
If it's like season 1 and 2, the price will drop from 90 to 60-ish on the Sunday before release.
For season 2, I didn't pre-order on Amazon until the price actually dropped, and as a result, it didn't ship until release day. I ended up buying it at Best Buy on release day and returning the Amazon copy, which arrived Thursday.