"That's insane. It should've been $13 with a blank line OR $18 and no extra blank line, in my opinion."

Other people disagree with you. It's quite easy for you to not add a tip than it is to have the room service attendee go back downstairs, print another bill and bring it back up to the room.

An easy solution for this is to simply ask the room service staff who show up at your room if this includes gratuity. They will say yes it does. What you should remember though, is they only get a small piece of this. It goes into a gratuity pool that also lines the pockets of managers who sit in offices and don't give you direct service.

Personally, I will often add a few bucks, but keep it in the 5-10% range.