Invitation Letter

Hello, I am applying for a tourist visa for 5 days to visit a friend and we will visit 4 different cities, with a valid GBR passport.

Do I need to include and invitation letter with my application from them or is a details travel itinerary sufficient?

I will be arriving from/to Singapore to Shanghai and departing from Shenzhen so the 144 hour visa is not applicable.

Thanks

Oli :)