Throughout the first half of the episode, several of the characters (Rory, Cate, etc.) mention that Paul was going to the store to buy milk and that is when he collapsed. When Cate comes into the girls' room to change at the end of the episode, Kerry says that he saw that they were out of batteries and that was the intention of the store trip. While it is possible that he was going for both milk AND batteries, it seems unlikely due to the nature of the dialogue that is used, and is more likely an unintentional error.