Staying in Apartments instead of hotels

Hi,

We are a family of 5 (2 adults and 3 kids) planning to visit China this summer. We are planning to visit for 14 days and we would like to stay in apartments using services like airbnb or wimdu instead of staying in hotels at least in big cities like Beijing and Shanghai. My question is do I need invitation letters from the persons that I'm renting the apartment or just showing that I have paid for the apartments for the days staying there enough for applying to tourist visa?

Thanks for your help