I order from the U.S. quite often. I pay now for what I order and it is sent to me. I have ordered a couple of times where there has been an item on back-order. When it arrives the back-order item is charged to my credit card and sent out. I pay postage both times. I have received an email to let me know the back-order item has been charged and shipped.
I don't have a problem with this and don't know why anyone should. With Canadian customs if the order is over a certain amount we have to pay duty on it. Shipping on the back-order item is usually cheaper than the duty I would pay if everything is shipped together at once.