[Starscream Gaga]

I think I've cancelled 2-3 items from Amiami, just send them a polite email and explain why, they're pretty reasonable. If you're worried about having everything cancelled say something like "I have other pre-orders with you that I will fulfil, so if this results in the cancellation of those pre-orders, please do not cancel any of them". I said that once and they just cancelled the one I wanted and let me keep the others. Just be polite and I'm sure you'd be fine.
I think it's a different situation if the items are in-stock, but you'll be fine.