Piro speaks:
actually, its not a mistake on my part - i know that in Japan its unusual to tip - but i feel like making the concept of the Anna Millers shop such that the girls DO get tipped by the otaku who come there.
doing such stories requires a little poetic licence at times ^_^ the other joke is the concept of well built waitresses - easy to do in anime, not always the case in real life in japan.
layer upon layer...