Yea because they forgot to book their spots on the ferry for the summer 
ROFLMAO at that!
Oopsie
It was pretty funny! “What can Brown do for YOU?” Well…deliver my packages for one…
If I were Fedex I would be trolling UPS nonstop with photos of Fedex trucks on the ferry 
Meanwhile these are a thing:
Oh wait…someone beat you to that too!
The summer population of Nantucket is substantial - and composed of folks who are well & truly addicted to Amazon, etc. I would imagine that the savings achievable by bypassing the need to pay for numerous daily trucks on the ferry would be substantial. Also, those delivery trucks would be working delivering stuff instead of hanging out on the ferry 
.
For almost all deliveries, the last leg of local delivery of Amazon is done by USPS. With or without Amazon packages, the mail is still going to the island.
Interesting - and definitely not true where I am right now in SW Florida. Between us, we get at least one Amazon delivery a week, and pretty much none come via USPS - here its mostly actual Amazon trucks, with “private contractors” during the holiday rush.
It could be the USPS wants too much for the delivery contract on Nantucket, so they’re seeking other avenues.
At least UPS can use a truck on Nantucket. Meanwhile on Mackinac Island
