If one invites a provider to his hotel room,
what should one do to avoid having her take
the money and then say she needs more cash
before she will provide the services we all
know she came to provide.

If one has the misfortune of having this happen,
what, if anything, can he reasonably do to
either receive the service at the expected price
or receive a refund.

You are posting this question on TER?  You already know the answer.  Do your homework and read reviews of the provider.  If she is listed as upselling, don't call her.  TER is the greatest resource but it is only great if you use it.  I do my homework and it has paid off.  The reviews are so accurate that when I see the provider it frequently feels like deja vu.  I mean that in the best possible way.  I read about what happens in a session, think it sounds like something I would want, make the call and a couple of hours later it is happening to me.  I see you are not a VIP, that is a problem as you can not fully take advantage of the resources without it.

Do your homework!  When I first joined TER as a newbie back in '04, I read posts on the NYC, Newbie, General, and other boards where I traveled.  Read numerous reviews, paid for VIP, and read the juicy details of reviews.  When I eventually picked a provider to see on a business trip, her reviews were mostly good...and she contacted me on the night I returned home.  Not a good start!  I then went to my home board...a superstar lady posted that she was visiting.  I e-mailed her that night...and met with her during her visit a few days later.  The four year pussy drought ended!  

The rest is history...still see that first provider, and the last four + years have been empowering, albeit expensive.  Avoiding upsells is easy...see only the top rated independent ladies, and stay away from agencies and CL!  As for droughts since then...longest has been two weeks!

If you have the misfortune of her already there with your money in hand there really is not much you can do, time to just count your losses. As others have said, you need to do your homework to avoid situations like that. I will comment that the advice about avoiding agencies makes no sense at all to me, some of the very best providers out there are agency ladies, you just gotta know who you are dealing with before they get to your hotel room and get your money.

there is little you can do at that point.

The advice that you recieve here will help you avoid ever being in that situation in the first place.

Pay the provider at the end of the session. It takes a bit of skill to get the provider to go along with it , but it can be done. You can also start a conversation about what you expect from the session prior to paying the fee. Many providers want to see an ID then they want the fee. The time between presenting the ID and providing the fee can be spent talking about the session's expectations.