It just gets confusinger and confusinger ....
What we do ...
Whoever ratchets out of bed first and doesn't hit a bulkhead opens the door and hands the coffee person $2. The other one (that would be me) hides under the sheets and makes nasty comments about the time of day people annoy me with coffee. Never mind that I was the one who filled out the little door-hanger specifying the time.
If we just order sandwiches or a salad, about the same.
For more complex stuff ... lunch on the balcony, a special fruit and cheese or veggie tray for cocktail time, etc. we part with $5.
If we decide that we want the entire dinner thing, ordering from the dining room menu, done "en suite" and the server does a good job, that's worth a bit more.
If we happen to have a butler (a dying breed, apparently) we just tip the fellow at the end, keeping track of what he's done.
We do tip the dining room waiter for EVERY night, regardless of whether or not we've been at table. He was there, we weren't.
Isn't THAT helpful?