In the story, Patty lies about what Anton Reiker bought because she wants to protect him. When the FBI agents question her, she tells them that the only things Anton bought were a 'sharpener, paper, and pencils.'
Then, remembering that Anton had bought a faux diamond pin for a dollar, she resolves to cover his tracks by saying that he bought a 'straw field hat like the rest of the prisoners.' Why does she do this? To find the answer, we will need to refer to Chapter Eight, where Anton confesses to how he used the pin.
In effect, Anton utilized the pin to buy his way out of prison. He explains how he bribed a prison guard by using the three-layered approach Hitler used to manipulate his audience. Essentially, listeners can be lulled into security by the first two layers of fact-filled truth; the third layer, however, is the one that holds the lie. So, Anton's excellent English allowed him to convince the guard that he came from a wealthy background; his assertion that he had had an English governess further reinforced the exclusive image he wanted to emphasize. From there, it was an easy task to convince the guard that he had five diamonds, valued in excess of a thousand dollars each, to bequeath anyone who helped him escape. Basically, Anton paid the guard with 'a dollar's worth of glass jewelry.'
Knowing this information, Patty does not want to tell the FBI agents about the pin Anton bought. Because of her love for him, she does not want to incriminate him.