Olivia M. answered 03/22/19
Experienced Tutor Specializing in French And English
Bonjour! After living in France for the past 6 months, I feel that this is not easy to learn without proper context. What I mean is that, when not using it everyday, it tends not to make a lot of sense. I'll do my best to answer all of your questions!
As far as "Bonjour/Bonsoir", this is your greeting. Just as in English you would say, "Hello/Good morning/Good evening." Whereas, "Bonne journée/Bonne soirée" is your pre-goodbye. In English we would say, "Have a good day/Have a good evening!" this is their usage.
As far as the time of day when to switch (I've legitimately asked this question and no one has known a formal answer!) I just gage it with the sun and others might have a different gage. When the sun is setting is when I think to switch to Bonsoir instead of Bonjour but there have been many time where my "Bonsoir" is met with a "Bonjour" (very confusing, I know!) Sometimes a greeting is just a greeting, though the French tend to make a big deal over nothing. Another rant for another day...
As far as what to say after seeing someone again later in the day, (I've only learned this just recently) you could use "Rebonjour" or "Rebonsoir" (no, I'm not joking). It is literally saying, "Hello, again!" which I use often in English, anyway. My go to had always been "Salut!" (and still kind of is, old habits die hard) but they prefer "rebonjour" or even just a quick "re-" in a very casual setting (again, no joke).
Lastly, as far as your order between "Merci", "Bonne journée/soireée", and " Au revoir", this ordering is my go-to but any order really is fine, I've heard them all from others and shop workers.
Hope this helped! Bonne journée!