If you walked into a sting it is not only possible but pretty much a guarantee that you will be arrested.  It will happen before you actually start "going at it".

As far as LE is concerned answering an ad for prostitution and showing up with an envelope full of cash are enough to get you a ride to the police station.  Conviction is another story but your personal life and career will pretty much destroyed simply by your being arrested.

100% guarantee for anything, other than death and taxes.  You have done what you can do to minimize a problem with LE.  

What many hobbyists do is enter her incall, she may give you a hug or a quick kiss, then close the door.  Put the envelope down, turn your cell phone off (a hint for her to do the same) and put it on top of the envelope, then put down your keys.  She may ask you if you want a drink or ask you to sit down to chat.   NOTHING is ever said about the envelope and/or why you are there.  She may ask you "What do you like"?  A safe answer might be "Most anything, why don't I start by rubbing your back?"  Just be a gentleman (especially in your choice of words) and all will be fine.  Let the flow take you where you need to go.

I now do outcall as it is a bit safer...le works mainly incall

The cops have 2 main goals.  1- for the individual cops, meet their arrest quota 2- for the department/ DA, to supress the Hobby thru harassment.  They don't put you in prison for decades anyway, they arrest you, demean you, emberass you, take your money, force you to jump thru hoops, appear for trials, all to deter you thru harassment.

So, if you show up in their trap, and are careful to avoid even a nodded head or grunt to any soliciting question like 'you here for the GFE?' or 'wanna get started' or whatever, because of 1 & 2 they'll bust you, even if you could defeat it in court after going thru all that trouble.  BUT, remember, even to show up, with the correct amount of money, EVEN in your pocket!, in response to a specific Ad, after calling that Ad's number several times and following instructions to find the room, COULD BE evidence of soliciting.  If you were smart enough to know what the transaction offered was, LE is too, and the rules of evidence changed on this in the late 90's, allowing LE to use common sense.

Bottom Line, you show up to that room, you're toast.  Envelopes don't help, calling it a 'donation' don't help, not talking about the $ don't help, a tit or balls grope LE check don't help, using euphamisms like GFE or Fun or whatever don't help, that's ALL a MYTH.  

You Show Up, You Get Arrested.