It seems like North Shore really went above and beyond in helping out and they don't make companies like that much any more these days!! So North Shore is definitely a keeper!
Another thing to remember is that Martha's Vineyard is an island with vehicular ferries from two places on Cape Cod, Woods Hole and Hyannis. There may be other places on the mainland but they may not be vehicular ferries. And because of that shipping can be a problem. In fact, people visiting either MV or Nantucket are encouraged not to bring their cars if at all possible (and just use local transportation) because the ferry service is so popular in the summer. But FedEX and UPS trucks may be given some priority space on the ferries. And with just two bridges going over the Cape Cod Canal connecting the cape with the rest of Massachusetts, the traffic jams, especially in summer, are enormous!!
Here's another idea: if there is a gap between your delivery on MV and when you run out of whatever you can bring from home, I'm sure there may be at least one medical supply store on the island. And they may be able to provide something to tide you over until your delivery arrives. There are still a lot of permanent residents there so when there is severe weather and ferries and planes aren't running, the people still need medical supplies so there must be such a place there. I guess you can find out if you Google "medical supplies Martha's Vineyard" and email them or contact them when you get there. I hope this helps and that everything arrives on time, or "seamless" as they like to say!