Factual error: The movie leaves the impression that Huntsville, Texas and Austin, Texas are just a short distance apart. In reality, they are at least three hours apart.
Factual error: When Bitsey enters the prison for the first time, the sign reads in Spanish "Cierre la puerta fuerta." It is misspelled, it should read "fuerte." The word "fuerta" doesn't exist.
Factual error: At the end when Kate Winslet receives the Fed Ex box she opens it easily from the end. Fed Ex boxes have pull tabs that you pull to open, and if you decide you want to open the box at the end it is not as easy as she made it look.
Factual error: When David is reading the e-mail about his son's custody sent by his wife, the date reported on is Tuesday, 8 Oct 1994. However, that day was a Saturday. (00:48:39)
Answer: I believe that she was a plot device to make sure David Gale had nothing to lose by the end of the movie. She then may have felt guilty about casual sex, and tried to get back at Gale. She then later felt guilty about the whole charade, and wrote him a postcard admitting what she had done. Then the postcard is given to the ex-wife to show he was innocent of rape.
Richard Welty
David had asked Bitsy and, presumably, Dusty, to preserve his son's memory of him. So they/he/she forwarded the postcard from Berlin so his ex would know he was innocent of the rape charge. Along with money for his son.