On this option, my TA also would have given us a discount on the final Princess price so going through her would have been the best deal by far had we chosen to book a full suite. I alternately checked the upgrade price to book a full suite and that cost exceeded what the list price minus the normal casino discount would be.
We just booked an Antarctica cruise for next year through the casino upgrading to a mini suite from the free inside cabin that the casino offered. When you do that you only have to pay the deposit immediately and don't have to pay the balance until the normal final payment date. What you may want to do is check in with your TA or Princess and ask them to provide a quote with just a casino discount.
Yes, it is normal to have to pay the upcharge and everything else at the time of booking.
