We all run our business in our personal way, I suppose.
In this particular aspect, most of the times, since my appointments are planned well in advance and last fairly long time, I take the envelope without even opening it in front of the guy.. knowing that, well before the meeting is over, I'd find a discreet place and time to check if the anount of money is actually what we agreed beforehand.... So far every single time it did, never had any problem, thanks God!