you ideas about how I prefer to handle extensions...

In your first example, when you're already on the date and having a great time, as the other ladies have pointed out, just ask if she has availability to extend. If she says yes, please give her peace of mind by immediately adding the proper amount to the donation. That way she can relax and enjoy the added time.

In your last question, you referred to booking a 2 hour date with the option to extend. Since I do entertain mostly on tour, I will tell you that requesting an option to extend puts me in a difficult spot. When you book a set duration, I know I will be done at 4:00, say. But when a client wants the option to extend, now I don't know whether I'll be done at 4:30, 5:00, or 5:30, depending on my client's desires. That makes it very difficult for me to book another date afterwards, and may actually cause me to turn down a date who can only come at 5:00, say.

This scenario actually happened to me recently in Charleston, and I missed another appointment because my "may extend" client came in firing on all cylinders and burned himself out in 45 minutes. By the time I was free to shoot off an email to the second client, it was too late for him to make arrangements to visit.  

So, something to consider, from a lady's point of view. Thanks for asking the question!