is it customary to tip a provider?  if so how much?

...it's appreciated but not expected. Consider how much they get for an hour. It's ok if you believe they went beyond expectations.

Use the search function here. You'll find many discussions of this. When searching change the number of days searched to 100, 200, or more.

The best tip you can give is a to see her again if you had a great time.  If I've seen someone on a repeated basis, I will review her website to see if she has a "wish list".  

Tipping really is a personal choice whether to or not, same with the amount of the tip

The ladies are not dependent on tipping and they are setting the rates they feel are appropriate for their services.   Just as I would not tip a lawyer, a doctor, an electrician, a plumber and so on, I am not inclined to tip a lady.   This is particularly so if this is a lady that I want to see again, not only because of the oft repeated line that seeing her again is a tip, but because of the precedent it would set.   For example, if I don't tip the next time, is she going to think that I did not enjoy it as much?   So, in a very real sense, if I tip a lady I want to see in the future, I feel that I am increasing her rate, and really, why should I pay more than the next guy?  

A long time ago, I used to see agency ladies, and not so long ago, I also used to see stripclub ladies (thanks TER and P411 for getting me out of that!).   When it came to agency ladies, I felt more of a sense of needing to tip, because I knew that the lady was only keeping a certain percentage of what I was paying.  Likewise, with the stripclub ladies I saw at the club, not those who would see me outside of the club, they are paying a substantial fee for their time in the VIP room.

In my case there are a few things that I think about as a tip.   I am not a fan of reviews, but alas, this is a review site after all, and as much as I don't like to share, I will do so partly as a 'thank you' to a lady I really enjoyed spending time with - the other part of the motivation is to contribute, at least a little bit, to the general cause.     I will also gladly reply to pms from other folks who are thinking about seeing someone I have reviewed.  Again, partly to thank the lady and partly to contribute to the cause.

Finally, I am also not a fan of the dinner dates and such, not at $200/h, $300/h, $400/h plus for the time spent in public, particularly if I have not met the lady on a previous occasion.    Even if I am traveling, which is when most of my dates take place, I can have a good time dining at the bar with other patrons.  However, if it is someone I have seen before, if I really like their company and if I know that I am going to make an effort to see them multiple times, I may certainly do a dinner date.   In the lady's mind, this would not be a tip, after all, they have to spend time with me and be 'on,' but it is something that I would not normally do, and as much as I may enjoy the company, it is also partly done to thank the lady

is like a $XXX tip.

Particularly with the agency part.   I still use agencies periodically and I do tend to tip for the exact reasons you mention below.