We often thank people for things they haven't done yet, as also seen in the different phrases "Thank you for your cooperation" and "Thank you for your understanding" when people are only just hearing that they need to cooperate or be patient etc.
I teach my students that in emails "I look forward to hearing for you" etc just mean you need a response, e.g. when they will be available to meet, but that is not enough for a big request like "Can I halve my order, even though it's ten minutes before you'll send the delivery and I made the original order three weeks ago". There is a danger that "Thank you in advance" sounds like a polite way of saying "Don't forget what I asked you to do", which is why I advise never to use it in spoken communication, and if at all possible use the more casual forms "Thanks" or even "Cheers" as they seem like less high pressure, just as long as you don't have to be super formal in that situation.
