Oh yea, I'm certainly not questioning whether it made sense for Amazon to buy up some planes, because they definitely ship enough volume (even just domestically) to experiment with that.
Though I would argue ships have advantage in both mass and volume, but are significantly lacking in latency.
In case I wasn't clear, it isn't just the latency, it is the number of loads that can be made in a given amount of time.
Just like a restaurant that is always full makes more money if their clients eat quickly and leave, freeing up space for more customers to do the same.
From my own supply chain experience, by volume air freight is only slightly more expensive than sea, but by mass the difference is an order of magnitude.
Though I would argue ships have advantage in both mass and volume, but are significantly lacking in latency.