I believe you are allowed £18 allowance or £36 if it's a gift (although this includes shipping cost as well so be careful) so if your order is under this you're okay; if not I'd go elsewhere. I reckon that over the basket of goods I've imported I've had to cough up on about a third of items over the duty threshold so there is also a lot of luck involved.
To your other point, I haven't found attracting duty makes much difference on whether a parcel is opened or not. I've had opened items below duty thresholds and unopened items that attracted duty.

