First time cruiser and hoping my math is correct. I have the basic package and it will cost me $840 to upgrade to Plus. So I take $840, less my $200 OBC, less $240 for my crew appreciation, less roughly $200 for wifi, I'm now at $200. But if I buy $600 of AARP gift certificates at 10% off I'm now at $140, is that correct? And to confirm, I can upgrade the day I board? Thank you!