My husband and I booked a private tour from Reykjavik with Tourism Iceland through Trip Advisor. Unfortunately, we could not make port in Reykjavik because of bad weather. Our captain reported this to us at 4 PM the day before we were to arrive in port. I immediately called both Tourism Iceland and Viator to report our problem and request a refund. They refused because I called after the 24 cancellation window had expired. Of course, we had no power over the port cancellation, but this made no difference. I strongly suggest that you consider other options in Reykjavik for shore excursions than Tourism Iceland. Tours By Locals offers insurance to cover eventualities such as itinerary change, and this is a much better way to go given the frequency of itinerary changes beyond the cruisers control.
