Maybe I'm an oddball, but I've never had a problem simply giving an escort a stack of bills sans envelope. And no SP I've ever seen has had a problem with no envelope. As long as they get the agreed on money, they don't care.
Yes, girls can get ripped off, but let's put the shoe on the other foot for a sec. What's to stop an unscrupulous SP from taking the envelope, counting it out of your sight, and coming out and saying you shorted her $20? Most guys would probably cough up the $20 rather than spoil the session by making a fuss.
I think in any commercial transaction, including this kind, it just makes sense for both parties to see the amount being passed at the same time.
I think the whole envelope thing is more in the U.S. than Canada, where prostitution is (mostly) illegal and so has more of a nudge-nudge-wink-wink aspect to it.