Tips, gratuities, etc.

Ok so you mentioned lot of different things here.

For the tips/gratuity, there are no common percentage. It's really up to you, your budget, how your session went, etc.

For the gifts or attention right before the meeting, similarly It's really up to you, your budget and how you want the session to go.

For the gift registry, I would say it's a good token of appreciation for say you asked lot of questions and she spent her time to answer it for example. But remember, it's more a kind act of gesture rather than something towards your envelope.
Or it's also a good way to show that you are serious and you exist (and you're not a timewaster or scammer) especially when it's the first time you meet.

If you are a regular the 3 above type of appreciation is something you might give because of the long "relationship".

I asked the booker what is the correct etiquette because it changes depending on culture. In the US, all service industries expect some tip. In Europe, not so much. And in Japan, it's insulting to leave a tip. The booker said basically you don't have to and most do not, but you can if you want to. It seems like when I've given above the rate, they don't usually give back change. But also, it seems that even though I would expect that most people just want money as I do at my own job instead of a pizza party, it seems that SPs are a lot more touched/appreciative of thoughtful gifts.

I thought it would be weird if I brought something like a bouquet of flowers after I bought it because I guess they could want to hide it from the next client or just dump them to avoid walking home with them, but one SP kept them and took them home.