I would guess probably not. The postman should not deliver it without getting the money, I would think they just didn't see the demand. I would also say that you could just deny all knowledge if they come after the money. After all, you just opened your package and threw the wrapping away, didn't you? Did you notice any demands for money? Thought not.
I just received the first and only game I will order from them this morning. Ordered Sonic Advance for someones Xmas present and got an £8.29 charge. I only got a ticket through the letter box, unfortunately.
It's not so much the customs charge I mind as the £3.45 included in it as the consignia handling fee. Ended up paying more for the game than if I had gone down town to Game. I will stick with DVD Box Office for games from now on. Wouldn't go elsewhere for CD's though.