I’ve used flights booked through Air2Sea successfully a couple of times. The things I like about them are:
(1) you can often get really good prices on 1-way international flights,
(2) you can cancel before final payment with little or no penalty,
(3) if the price goes down, you can contact the cruiseline and get the price lowered
(4) no additional money is required until your cruises final payment
BUT you need to be aware that those bookings are not like the ones you book directly with the airlines. They are negotiated pricing that have specific restrictions associated with them. You may never even realize it if you don’t have any problems with the flights or connections. However, if something goes “sideways”, those restrictions may become very evident. As has been said multiples on these boards: Caveat emptor.