Don't know if this will 100% answer your question, but I've never had an item on preorder that, when processed/charged, would alter the shipping to bring the entire cost of the postage down below what you'd initially pay. In fact I've often had to pay an extra few dollars to account for the new stuff in my order (when I've processed Pile of Loot stuff).

I think you can grab them on enquiries@bbts.com or customers@bbts.com, so maybe give that a go? They're quite quick/happy to explain everything.

best of luck!