You are encouraged to not wrap anything and if you do, the US customs officials reserve the right to unwrap them (and if they need to do so, it will delay delivery). I've never had any of my family/friends in the US charged any duty or tax on anything I have ever sent.
BTW, you've just missed the Royal Mail deadline (it was Friday) for delivery before the 24th to the US, but as they only have to go to NY, you may still be ok if you get things out today.