...that the best tip is scheduling another appointment, and I believe that virtually every provider feels that way.
Other things you might consider while contemplating the question:
Agency girls give a sizeable cut of their rate to the agency, but keep all of any tip you give them.
Massage girls in a shared setting, same thing.
Providers with schedulers/assistants also give up a cut, though I imagine it's a lower percentage than the first two examples. On a couple of occasions, I've extended my appointment with a provider during the appointment. My imagination tells me that she got to keep all of it. Effectively, a stealth tip.

I tend to round up. I saw a provider who listed $275 for an hour, but that was too complicated for me, so the envelope had $280. Now that I'm on my third visit (an surely not the last), I just use 50's to round the hourly or 90 minute rate up. She's never mentioned it and I would feel odd if she did. (Why?)

I also once saw a provider who listed a P411 discount, but I didn't take it. It seemed like a great value without the discount (is that too "marketing-speak?) and it absolutely was! I felt like it would have been cheap of me to take this discount, but maybe that's the little head thinking. If I stay much longer in this hobby, my big head (and bank account) may take over.

Is it a tip if you pay ahead of time?

It's entirely up to you.  It's true, like most have posted above, that most providers welcome tips but do not really expect them. I have read on this site that many providers consider a repeat visit the best tip.  Me, I always take a gift, usually candles and if I had a really good time, I will book another date.

LLAP,
Swim