Ahh! My bad. I thought PO to be Purchase Order. Nevertheless, I have read about some people taking delivery of cars without invoice, and later suffering at the hands of dealership for not getting invoice on time. I think the creator of this forum @pakwheelsimb once suffered from the same thing. Let him confirm this.
Also, how can they offer you the discount of 20K, if the car has still not been invoiced? Had the car not been invoiced, the dealership could have sold it off with a 2016 invoice (without giving any discounts)? The discount explicitly suggests the car is already invoiced.
So, my humble advice would be to not take delivery before they give you the invoice in soft form at least (Like they sent me via email) to avoid any unforeseen calamity later. Best of luck with your purchase.