I will try not to color this with any rude undertones. My intention is not to be rude, but more matter of fact:
You placed an ad on an escort mall, have a website ect, ect. The client sets the appt up and shows up. It doesn't matter if you have him mark, "Donation to the sister's of mercy." or whatever cleaver thing you have him put on it. A meeting of the minds has been met long before he arrived. If he is LE you're going downtown, end of story. I personally don't touch the envelope in the beginning because I think it's tacky not because I think it will keep me from getting arrested.
The screening advice is good, as well as not having explicit conversations.
You have given advice on how to avoid legal problems. The OP was about the provider running into an essentially abusive type of man.