Unfortunately, Air2Sea is nothing more than a consolidator. Could very well be that the restrictions on the ticket when negotiated don’t allow much wiggle room for changes.
Air2Sea air travel tickets are NOT the same as a ticket you would buy direct from the airline. In fact if you buy more than 1 ticket, each ticket could have different restrictions.
99% of the time, you’ll never know the difference. But, sometimes, when flight operations go TU,,,,, problems.
Of course a good marketing plan will never tell you this.