They are looking to develop regulars who are gentlemen with class, and therefore do not expect you to pay up front.

You could set the fee down at the beginning, but I doubt she will touch it until after you leave.  

If you can afford it, then by all means, give her a tip.  Not everyone does, although some are very generous that way.  

If you want to be sure that she knows why you are tipping her, then put the tip in her hand afterwards and tell her specifically.

Good rule of thumb is: "Business first, pleasure later," no?  I always get the $$ out of the way first so that we can both relax and just have fun.  If you are not fully at ease with paying up front I'd suggest telling the lady that since it is the first time you would prefer to pay hour by hour, i.e., $ or so for the first hour and then more for the second and third and ??? hours.  I usually pick up the cab fare - $5 - and give something for the booze or beer I drink plus a tip at the end.  I've seen five different Internet TJ ladies and never been disappointed.  Buena suerte. Lencho

Tips are not required, but always appropriate for superior service.  Like Lencho I always pay the cab fee.  Unlike Lencho I pay at the end.  (The business custom in Mexico is to pay when service is finished.)  I will admit that Lencho’s policy could have save me some trouble once.  Normally the girls aren’t clock watchers; however, I once had two hour session that went about fifteen minutes over.  I gave the girl $260.00 and she asked for more as she though we’d been there three hours.  I have to give her a math lesson so understood we’d only been together two hours.  I give her a little extra, but it was not the best ending to a session.  (I’ve seen several Internet girls and this is the first that watched the clock.  She also had a lot of “I don’t do that” rules, another rarity among the Internet providers.)