Next step is the Reconcile tab. Hopefully they'll investigate and suddenly find them or reimburse. There's a good chance you'll get the inventory.
I have, on multiple occasions, resorted to attaching photos of the box contents with the label in the picture to cases asking for reimbursement if they don't properly reconcile after another week or two past the reconcile date. It has seemed to help (or perhaps it's just the re-opening of the case again and again and asking them to check again). I have, on multiple occasions, failed and wound up losing sets that I know were in the box with no reimbursement. On the other hand, I have also had them over-count boxes, tell them "unexpected" in the reconciliation, and even open a case saying those were not in the shipment. Still, they stuck to the counts and once I sold the sets -- low and behold -- they couldn't find them to ship to the customer and wound up reimbursing me for them. I suppose it kinda works out.
However, the worst was when I had a couple large boxes come in for Christmas in the fall (50 pound limit for non pallets!) and they failed to receive something like 95% of the inventory (like you). Eventually back and forth with reconciliation, had to provide receipts, and they reimbursed me a few weeks later. Then -- and this really takes the cake -- they "found" the inventory after Christmas, deducted the reimbursement, and I was left with loads of Advents, Holiday sets, etc in January or February.
It blows, but, cost of business with Amazon. Hopefully they reconcile!