Being pedantic, it's not import duty you are paying there, but VAT.
Having said that, like you I have no issue with paying VAT (after all, we pay it on most things we buy but we don't notice it as it's folded into the cost) but the £8 handling fee really rankles with me too.
Having said that there is some more administration than you mention: inspecting the data about the parcel, calculating the VAT, making the entry to HMRC's sytem, producing the card, receiving the payment from you, accounting for and reconciling the payment, (double/triple) handling the parcel itself though a different channel than 'normal' mail. One could even say that £8 is a bargain price for that - but it certainly doesn't feel like it!
It certainly doesn't feel like it when the imported item is of modest value.