Three ways that the humiliation still comes through. One way is that the sub 'forgets' about the fact that the Mistress is doing what he asked her to do. I think this is a very common aspect and practice in BDSM paid sessioning.

Two is that the Mistress laughs at the very real fact that you want her to shit on you and even paid her to do so. I think many would consider her words to be humiliating.

Third, and I believe that you alluded to this, is that she creatively adds on to your request. For me, a memorable session involved the Mistress having me lie there, covered in her feces, while she carefully freshened up herself, in no particular hurry.

Then she had me get up, still streaked with her waste, and I was made to wipe her ass spotlessly cleaned, being reminded how important it was that her ass be spotless. Never mind that I still had her shit on my face and wasn't allowed to do anything about it.