A friend of mine had a bottle of beer break while it was being shipped via USPS. They called him in to an office at the post office and they just said "most people don't realize it's illegal" and handed him the unbroken beers.
But I wouldn't gamble on it...
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I'm sure it's like most less than obvious offenses, where the odds of anything other than a warning are pretty low, but like you said, I wouldn't gamble on it.