I would like (NEED!) this feature as well!  Yes, I know customers can choose to increase the payment amount manually, but it's awkward and does not "prompt" them to tip.  Prior to using QBO i used Square for my invoicing, which has this feature built in, and received a lot of tips that way.  Now, since switching to QBO that income in virtually gone.  ITS A MUST ADD FEATURE in my opinion!