The Sliding Cargo Deck is a Factory installed option. You can get it ordered like that if you want to wait. To my knowledge, the sliding cargo deck doesn't get installed at port. Only the true bolt-ons are added at port (e.g. The TRD Pro badging, exhaust, TRD wheels, roof rack, and port options like emblem kit, predator steps, etc.
Once it leaves the factory in Japan, it's too late to get the sliding cargo added. You can, but likely not at the dealer.... you'd have go through the hassle of buying the parts needed and installing it yourself.
[MENTION=402281]violane[/MENTION]: Since you have your price sheet, you can use the VIN# there and enter it on your Toyota Owners profile online or through the app. From there, view the vehicle specs and it will show you that the cargo deck is a Factory option.