Invite Letter for L Visa in Hong Kong

I'm currently in China and will travel to Hong Kong for a tourist L type. I will have an invitation letter stating they will sponsor my stay here during my time. Is it possible to use the Invitation Letter in lieu of travel itineraries, such as round-trip airline tickets and hotel booking? Or do I still need to show airline tickets? Also, without airline tickets, is it possible to apply for 60-90 multiple entry options or just 30-day multiple entries?