In this specific situation as the OP's this is my stance on it as well.
I mean really how many times have we all used someone else's driveway to turn around in? I really try not to do it out of courtesy but sometimes there is little other option. If this was a one-time thing of a neighbor driving their car onto my driveway to get the mail I wouldn't let it get to me but a habitual thing is something I wouldn't want going on.
I would approach the neighbor quite nicely and ask that they get their mail without driving up on my driveway. No explanations should really be needed.
We had to talk with our neighbors about their kids using our driveway without our permission as a sledding place and a place to ride their bikes down--they would get all the way up to our garage doors:
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First few times they did it we didn't let it bug us but they kept doing it so we talked with our neighbors on either side of us and they in turn talked with their kids and it hasn't happened since and we sill have a great neighborly (sp?) relationship going on.