-shower is counted as part of the time, yes.
-It's best to clear up what services you want before the meet/at the start - obvious ones shouldn't need discussing so don't pester her with questions, but do make sure you both know what's expected e.g. if her profile doesn't make it clear if she does CIM or not, but you want that, just ask if it's available
-there's no 'code' or anything with the money as far as I know. Either she'll ask for it or just hand it to her. I'm still new to this as well and have taken money in an envelope before because I thought it was expected, but the girl just laughed and said something like 'ooh very professional'
so I don't bother anymore. On my last one we both forgot about the money until the end..
-I don't have specific tips for nerves, it happens to everyone. Just stay calm, remember you're going there for a good time and your nerves will fade quickly once you're there anyway. Definitely don't drink before as you run the risk of her not going ahead with it (or some services) if she suspects you're drunk or can smell it on you if you have a lot!
Good luck & remember to review it