Thinking of seeing an agency girl.  From reading the posts it seems it is common place to tip these girls since their rates are cheaper and they split it with the agency.  Now I usually don't tip an independent girl.  So how do you give them a tip when it seems taboo to hand money over to the girl.  I really don't want to include it in the envelope in case the session is a disappointment.  

As you hug and kiss good bye, run your hand down her arm as you back away and when your hand is against hers, she will feel the bills and know exactly what is going on.

It works like a charm.

There are a lot of shady deals in Vegas. Find an agency who's girls have reviews and be sure that you re not getting involved in an up-sell scam.  

If you want to tip at the end of the session it's not a big deal. Leave the money on the dresser or hand it to her and say thank you for a great time.  Once the session is near it's end and she is sure that you are not a cop the anxiety regarding money-handling is gone.

...assuming neither of you care that it's not exactly GFE.  There's certainly no LE issue at that point.

Agree with Mr F, to go a step further be prepare with bills in your key pocket.  Easy to get it when the time comes.