Thank you! I suspect my mis-attribution of speech bubbles was a large part of my issue with understanding this (which also affected my translation, because I was trying to make them make sense).
It was indeed. So, "ちゃんと貰った手紙には返事しなくちゃ駄目よ。 まだそんなにあるんだから。" would be more like, "You should reply to...