Why can't I add more/different items to my cart with preorder items?

We do not allow for items from separate preorders or for in-stock items to be placed in the cart at the same time. This is because there are different shipping times for the products. All the products will be shipped out at different times, and we can not hold back orders.

This rule can also be in place for the same theme of group buys. For example, the deskpad preorder for "x" keycap set, and the same keycap set group buy can not be placed in the cart together.