I don't tip during the first date, and you wouldn't include it in the envelope in case the service was crap, and it sure would look shitty, awkward, or could potentially lead to an argument if you decided to take a tip back from an envelope.
If you're going to tip then tip on the second date; presumably you're repeating because you like the girl and had a good time with her the first time around so there's a good chance you'll like her the second time around too. Don't keep it in the envelope for the reason I mentioned above, and since you've already met her she knows you're not a cop and you know she's also on the level so you can dispense with the "it's a donation for her time I'm seriously not paying for sex guise" routine. I just place the tip on top of the envelope after the session is over if I feel a tip is warranted.
As for percentage, I have no clue, I just tip an extra $20-$40. I don't know if that's a good tip or not but fuck it, I think she'd be appreciative of an extra $20-$40. Or if not, let me know girls, is $20-$40 bucks a good tip or am I being too stingy?
I haven't bought gifts for providers at this point, I don't know the ones I've repeated well enough to do so, and I'm not going into a hotel or motel room carrying flowers or chocolates. Sorry girls, maybe on an OTC date. ; )