How does one ask a provider about donation and time for a session with multiple services ie: fbst w/ golden shower and pm, and when is the most appropriate time to ask?

For this I would look into her reviews. If she specializes in this fetish practice it will reflect in her reviews. A typical escort probley will not do these fetishes.(just fyi)

A GOOD provider will be all inclusive. She will not upcharge HOWEVER, some will ask a special fee for fetishes. In which you need to ask what fetishes she entertains and what the donation is. If she has a website I suggest you look there carefully before asking her. She might already answer your question. If the website does not you can inquire "I was looking at your website and was wondering what if any fetishes you cater to?"

can also PM other reviewers to ask them questions.

If necessary, a nice polite e-mail about "any extra fees for fetishes" might work--once you have been screened and have developed a raport with her.