We can only combine orders for a lower shipping fee if all of the items in your order are in stock at the time of placing the order.
We cannot combine a preordered book (a book which is not yet published) with a published book which is in stock. In this case, each book will ship separately. The in stock book will ship immediately, and the preordered book will ship when it is released. A single order may be placed at the same time though for both in stock, and preordered books. They will just ship separately.
The same applies to different preordered books which are not yet published because they are all released at different times, and we ship immediately upon publication.