Why did Jamie put "Massachusetts" for the post office in "From the Mixed-Up Files of Mrs. Basil E. Frankweiler"?
On the surface, it would appear that Jamie put Massachusetts for his and Claudia's address on the post office box application because he wanted a city unconnected to the circumstances of their real lives. Claudia and Jamie want to remain anonymous when they send the Museum their letter containing a clue to the angel statue's history because they don't want anyone to know they have been living inside the Museum. For this reason, the children go through great lengths so that their identity will not be suspected. They open a post office box and disguise their handwriting; since the Museum is in New York and Claudia and Jamie live in Greenwich, Connecticut, Massachusetts might seem to be a safe address to put down which will not call attention to their real circumstances.
On closer examination, however, there is another reason that Jamie apparently chose to write down Massachusetts, or, more specifically, "Marblehead, Massachusetts", on the application. Jamie is choosing marvelously humorous, creative names referring directly to the statue in question in opening the post office box. He signs his name "Angelo Michaels", in a play on the name of the purported creator of the statue, and in using "Marblehead", a real city in the state of Massachusetts, makes a delightfully tongue-in-cheek comment on the nature of the make-up of the angel's head (Chapter 7).