[quote name='Japakneez']I doubt that works. My online order page normally shows the discount on the eligible item. But when my item(s) ships the packing slip will spread the discount onto each item in the order.[/QUOTE]
The thing is these are pre-orders. Unless they've implemented something new into their system which disallows pre-orders from being canceled, hypothetically speaking, people should be able to cancel the pre-order portion after their accessories ship. Since they charge you when the pre-order ships, there's no way they should be able to charge you an amount that your order history doesn't reflect.
However, this coupon specifically states "pre-order may not be canceled" or something along those lines. I don't know if that means the system won't allow it after the accessory ships, or if they won't keep the coupon's value if you cancel it before then. I don't think they'd be allowed to charge you the difference or anything like that if you canceled it. Besides, there's nothing preventing people from just returning the pre-order game when they get it.
tl;dr- I don't think they can charge you more for the accessory if it ships at a separate time than the pre-order. And even if it does, people have been complaining that none of the coupons work on pre-orders, and there are some good titles on the list.