In your scenario, you have already paid for the expected/promised service (with or without envelope). If you think you should tip ahead of time to get it, then you have picked the wrong agency or girl.
The whole point here on TERB is that this is an honest, straightforward commercial transaction with an accepted price for the service offered that pleases both parties. If you want special treatment, or some unique service by all means say so up front, and you can certainly suggest a rate or price you think is reasonable, as can she. But call it what you want, — extra, upgrade, premium … — it's not a tip.
The situation is rather different in Massage Parlours, where customers have not been screened and there are license inspectors as well as police to be cautious about. Because of their licenses, even in that room with that stranger, there's some rationale for the euphemism, and the pretence that the money had nothing at all to do with anything untoward. Not the case with incalls.
Here on TERB, talking those incalls, no need at all. As with a fine meal, you tip for the experience. For the menu, you pay the prices.
