Never heard of this.  The universal code in any AMP I've ever been in was cash up front. Going rates vary by city and region. Also, everyone's idea of a "complete massage" will differ as well.  Some gals offer FS, some BJ, HJ.  

If you are talking about Indie FBSM ladies I would look for reviews just as you would with a full service provider..

Something along the lines of:  "Do you offer any extras?"  usually does the trick, and the answer is usually "yes".

Then you have to carefully see what's on the menu and offer to pay so much for such and such.

It's a bit awkward, but I've done it successfully many times.