I know a girl that will only do anal once or twice a day & yes charging extra will deter too many asking for it, just tell them that its only something you'd want to do X amount of times on a working day and then they will know to be your first anal booking the next time you are working.

I used to do it occasionally but only on a long booking ( not a long arse shag) but hey, now I quite literally can't be arsed