RM have referred the item (incorrectly) to customs far as I can see. Its down to them if to do this or not. As usual they are showing themselves to be useless.
OP: Whoever above said because the value is so low is correct, see
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageImport_FAQs&propertyType=document&id=HMCE_PROD_008654#P39_7232
It normally goes something like :
cost of item and postage
+ duty (usually 5-10%)
Then VAT on all of that.
Duty is based on a commodity code, there is a way of looking it up at the top of the page I have linked above.
But you shouldn't be paying it due to low value. I would suggest you challenge them on it, no point speaking to the drones in the offices, ring them up. The proper way to do it is here:
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageImport_FAQs&propertyType=document&id=HMCE_PROD_008654#P49_8401 but that looks like a good way to get lost in their red tape imho. You may have to go this way if they don't play ball on the phone