Should I greet my customer at 6pm with "good evening" or "good afternoon"?

Both "Good afternoon" and "Good evening" are perfectly appropriate greetings at 6pm. Pick one, and don't overanalyze it. If you say "Good evening" at 4pm, or "Good afternoon" at 8pm, you might get funny looks, but near the boundary, either is fine.

Good afternoon before sunset, Good evening after it?

I think this is largely regional. In many places, "evening" starts at dusk, in others it starts at "dinner time" (6-7pm?) or "after business hours" (5pm).

My personal preference is usually to say "Good afternoon" until around 5pm, and then I switch to "Good evening." But you can always just avoid the situation and say "Hello!" instead. :)

I think the "good x" greeting is (in theory) expressing a wish on your part that the other person is having a positive experience of x at the moment, and will continue to do so for the remainder of x. It doesn't really include past events.

So "good morning" at 10 am suggests "I hope you are happy at the moment, and enjoy the rest of the morning" but not "I hope you had a good breakfast".

Since by your definition at 6 pm the afternoon ends, you should say "good evening", because there is no afternoon left for them to enjoy.