I think I have seen both la douane and les douanes used for the French customs police.  Is there a rule for which to use and when?

I think in English, especially in the UK, it is an evolving language and many grammatical rules are being overwritten by common usage.  On that basis, I think it is becoming harder and harder to prescribe firm rules, and more often the answer is "either may be acceptable".  Unfortunately, bad/lazy/incorrect/slang grammar, used repeatedly, becomes acceptable/normal grammar.  I struggle to teach my children proper grammar, but they hear incorrect grammar all around them, even from teachers, and they use what the hear more than what I tell them is good grammar.  e.g. "James and me went to the cinema."