Each morning, with our continental breakfast, we signed a check to acknowledge receipt of the breakfast; on the check there was a box for "time received" (it always came in the time frame we requested) and another box for "gratuity". Whatever we put in the box showed up on our statement.
I also gave the room service steward a few dollars cash every couple of days.
The fact that you didn't upset the room service guys says a lot about Princess; that's the kind of attitude that keeps me coming back.
But next time kick in a couple of bucks.