[Timothee]

Instead of just driving to some far away place and come back, I'd pick a mailbox that's close to something where you might go sometimes but that's not close to your house either.

For example, you could go to Ikea buy something there (with a credit card to leave a log of a transaction) and mail your letter. Ikea because they aren't that many Ikea stores but it's not out of the ordinary to drive quite a while to go to one.

My point is that if you have reasons to want to send an anonymous letter, you might want to make sure that a drive that doesn't fit your regular patterns can be explained by something reasonable.

edit: I'm not sure the credit card is a good idea. Likely not. It might be better to pay cash, and keep the receipt. This way, you can't be immediately cross-referenced, but if asked, you have an alibi. Yeah, I really don't know. :)