Contrary to what someone said before the Royal Mail do not check every parcel, for a start they are not that efficient, and I should know as I own a Post Office.
My own recent experience is that I ordered up a pile of X-Wing stuff as it is not available here in the UK, for whatever reasons, the first parcel was around $100 at least, saved about £20, got through no bother. I quickly ordered another and it got hit, I see the point of the tax, but the £8 handling charge bites, savings this time fell to £4, but I got what I wanted. Almost the same thing happened with my son, I suspect because of the short time between these parcels the second in both cases 'flagged up?'
I sent for another parcel, this time around $30 worth after a fair lapse of time and it got through.
So unless it is a large box or of a substantial amount you will probably get away with it, but it's simply your donald duck!
My main reason for going to the US was that supply of what I wanted in the UK is terrible, the side benefit is that it is usually cheaper, despite the postage. I used an outfit called Miniature Market and I cannot fault them, quick, reliable service, at least for me.