If we assume that a) the 1h delivery is break even for them and 2) the 2h delivery would allow the courier to do a couple of extra deliveries on the same trip,

If we assume that a) the 1h delivery is break even for them and 2) the 2h delivery would allow the courier to do a couple of extra deliveries on the same trip, then I think the per item cost for 2h delivery could be only a couple of dollars, making it competitive with UPS and thus maybe free for prime members indefinitely.