How to integrate custom app with Sale Invoices?

Hi guys,
I’m working on developing a custom app for Hotel Management.
So currently i created 3 main doctype: Room, Room type, Extras
So when user reserve a room: it price will be the total of Room type price + Sum(extras price)
For Extras i can use the Items doctype which was already in Sale Invoice.
But in case of Room price, how can I put it in Sale Invoice ? since it can’t be an Item (right?)

Thank you guys a lot

You may add the room itself as an item ,
in stock module this item would be of group “Services” or something like that.