You should not have been charged import tax as your items were less that £105 which is the new limit that came into effect on 01-12-08
A link confirming this limit can be found here:
http://customs.hmrc.gov.uk/channels...cument&columns=1&id=HMCE_PROD_008654#P36_6903
This above link also explains how you go about claming back tax if you have paid too much.
By all means speak to RM and explain.
However if you are in a hurry for these items the quickest option would be to pay now and then claim the tax back.
You should however to pay VAT on your items.
As you've only been charged £12.25 I'd guess that is the VAT and the RM handling charge, in which case you've not actually been charged Import Duty.
To a post above - it's not a rip off, its simply a way to give UK businesses an advantage and the same thing happens in most countries.
If there was no import duty on items outside of the EU then everything would be bought from abroad and UK businesses would crash and burn.
You need to look outside at the big picture and not just at the annoyance of having to pay duty on the £150 worth of goods you might buy in from the US.