I have shipped small boxes (5-6lb) overnight with the post office, and cost is usually about $50-$60, have also used UPS and Fed X (slightly higher cost). What I like about the post office, is, if they don't get there by the time they are guaranteed to arrive, you get your shipping cost money back, by even an hour or 2.
With the others, there is no such guarantee, and they all contract with the same planes.
I have received fish airport to airport, and agree it is the best, because you usually get the fish the day they are sent (barring bad weather), and I believe the cost is comparable.
What's also really a bargain, is you can receive heavy boxes, and multiple boxes airport to airport for about the same cost. So I usually combine with another fish keeper, and we get a large order.