we aren't booking any tours independently when we are on the cruise, but we have had to book in advance for a couple of excursions we are doing along our drive - I think we have booked ones where you only pay on the day, so cancellation (on their part) will be disappointment versus not getting our $$ back (I hope).
We have only got one port for Greenland. At the time of booking we had three, and we have no idea why they got cancelled. The pay off is we are going further north up the east coast, so we are happy with that.