[shilala]

I've dumped tons of small flat rate boxes in the big blue mailboxes. Our blue mailboxes say the same thing, too. Never had an issue.
That doesn't mean it's okay, or that you won't have a problem, Randy. I'm just letting you know I've done it zillions of times over the years.
My PO is really cool and the guys that work there are awesome. If there's ever any kind of problem whatsoever, Joe or Matt will call me. Never had an issue with the little boxes in the mailbox and never had one returned. That said, it might be an issue in a larger more sensitive area, and I have no idea what the official policy is, or how the rule is written.
My guess is that it's a mandated rule and, as such, should be in the postal regulations. I know you can read them online, I've had to do that a number of times over the years in order to keep on top of bird shipping regulations.