Hi boxofsilver,
Thank you for sharing this situation with us. Honestly, I think you just got lucky with your earlier items being delivered without anyone noticing that money should be charged. All imported articles entering a country is subject for duty (plus any fees their country charges), and technically buying any item outside of the country, to be brought in, is an importation :)
You may also confirm with DHL what made the other items not subjected to this Custom duty fees (was it because of the price, was it tagged as gift etc).
Here's a link to Importation fees to expect in UK Link hidden: Login to view
Kindly update us on this, we'd love to learn. Take care.