For US Readers: Regarding Nail Polish??

At the risk of sounding idiotic, I was just wondering if the USPS restricts nail polish from being mailed internationally?  The UK Royal Mail does the last I checked, but other couriers are ok.  And I have received polish from China before.  The question for me is, there are a lot of US-based e-tailers that still offer international shipping options.  I ordered several months ago and received my orders, but then the next time I ordered, nothing.  And most US-based contests I see that involve nail polish as the prize are also not open internationally, but I didn't know if that was because of costs, or restrictions.  So if anybody knows definitively, I'd like to know as well.  Thanks!