I don't know of any evidence whatsoever that DeMohrenschildt purchased the Klein's money order. Do you?
Well we know that Lee didn't purchase the MO, And Marina is unlikely because she couldn't couldn't speak English... and she probably wouldn't have approved of spending their much needed money for an unnecessary rifle.......
PS.... As I recall there wasn't enough money available to Lee and Marina that he could spent over twenty dollars for that rifle.