If you have the item they can't charge you. They can hold it and only release it once the bill is paid.
Hmmm. I think that's just the UK? In Ireland customs don't hold anything, either the delivery guy collects the money off you or in the case of Fedex deliveries you're sent a bill later on. Just, its been long enough since I ordered with Sideshow that I can't remember how long after you get your item that that bill arrives.