I think you get the clients you advertise for, so I'm happy that flat-fee hunters don't call.
I charge for loads of extras, anything beyond sex and OW eg OWO, BDSM, anal, WS, no issues. It can bump up the total by an extra £20 to £100.
It works nicely and I know exactly what's required of me when they roll up.
I agree with Celeste things I don't like I charge an extra+ bonus.
This is clear on my website.