Help with a movie

Im looking for help with a movie that aired on USA Up All Night. The movie was I believe had a post apocolyptic setting but it was humorous, more a comedy than a gloom and doom. These two girls find that they are two of just a very few people who are still alive on Earth. Later in the movie they encounter this guy who at first is very wary of them and pulls a gun on them but then he comes to trust them. One of the girls who knows some stuff about weapons tells the guy that he will need a bigger gun with more stopping power. At another part in the movie the girls decide to go on a free shop till you drop shopping spree where they won't need credit cards or cash as they loot an apparently abandoned mall. They try on all these clothes and then a voice over the loudspeaker says they need to pay for their merchandise. They reach for their machineguns only to find that they're missing something and the voice says if they lost something they should check the lost and found. Anybody know what movie Im talking about?