When am I charged, and when is it confirmed?

It depends on how the guide set up the tour, and the tour page says which. Instant book charges you when you book and confirms your place at once; request to book charges nothing until the guide accepts.

Instant book. Your card is charged when you book, and your place is confirmed the moment the payment goes through.

Request to book. The guide reads your request first. Nothing is charged until they accept; then you have 24 hours to pay, and your place is confirmed when you do. If the guide does not answer within 48 hours, the request expires on its own and you are charged nothing.

What is charged is the whole price, or the deposit alone if the guide takes the rest in person. The tour page and the checkout both say which.