Never email Popcultcha! They take forever to respond, if at all. I find it's far easier to call, and any issue is typically resolved in a few minutes on the phone. The people answering the phone are quite nice, and I've never had any issue over the phone.
I called them today, and they said despite Captain Rex coming in, the Marty&Einstein set did not arrive in the same shipment. So they cant give an ETA on when that will arrive, since whatever supplier info they looked at also didnt have details. But the person reiterated that the preorder is still valid and all the preorders have been flagged as "urgent" so that customers will be notified immediately when it comes in.
So for now, it's just a matter of waiting......and waiting....