There may have been some very limited times/ships on which they were doing some testing when that may have been possible, otherwise that has generally been a paid option.
The D and D+ discount (can be used to purchase any available package) has generally been based on the cost of the 24 hr Surf package which currently is $26-28.
It depends on the fare booked - many of their fares include bags. If there's any fees for bags, that would probably be at check in between you and the airline.
I was trying to figure out what they are basing the D and D+ discount on with no 24 hr Surf being available. I recall the 24 hr Stream package was about $33?