Claudia wants to take a long bath so they can get a lot of coins from the fountain to pay for the post office box.
Claudia was annoyed that there was no place to take a bath at the Metropolitan Museum of Art in New York, so she dragged James to the fountain. The fountain turned out to be a great place to bathe, because it was full of coins. Claudia is thrilled.
We have income now. All we have to do is take a bath whenever we need money. (Ch. 7)
The children decide that they need to tell the head of the museum that they think the angel statue is authentic. They do not want to tell anyone directly, because they will get caught. So they decide to write a letter and give a post office box number for replies. Since the coins were their only source of income, they needed to return to the fountain to get money for the post office box. To rent a post office box for three months cost $4.50.
The children are very clever. The simple story is amusing and entertaining, and the reader is left wondering if they are junior art authenticators and if they will ever get caught living in the museum.