The short list of must mail drops are;
Port Clinton
Glencliff
Fontana Dam
Walasi Yi has a good selection of foods. No need to mail drop there in my opinion.
At NOC, I carried through from a resupply at Rainbow Springs
In Hot Springs, the outfitter has a good selection of food. No need to mail drop there in my opinion.
Damascus has stores. No need for a mail drop there.
Duncannon has a supermarket nearby.
I did a mail drop at the Water Gap, but could have used the shuttle to a grocery store.
Cheshire is a day from the supermarkets along Route 2 in North Adams/Williamstown. I did a small mail drop instead in Dalton.
Hanover has the Coop. The supermarket there closed. No need for a mail drop there.
The small store in Monson is adequate, and Mr. Shaw will shuttle you to a bigger store if you so desire.
That being said, I actually used several more mail drops to get maps and film.
Peaks