I've contacted HM customs in the past to get some clarification of their charges as I seemed to get some odd amounts charged.
The breakdown is as follows: Firstly we have a personal limit of approx £36 (goods and shipping cost) before charges are levied.
ALL charges are worked out on the declared value of the goods on the customs slip placed on the parcel by the sender or any invoice attached/enclosed.
To the GOODS value is then added the POSTAGE/SHIPPING cost. This is where insured surface mail for less urgent/heavier items becomes more important in working out what you are prepared to be charged
Import tax is then added to the combined GOODS+SHIPPING AMOUNT this is currently a flat 1%
On top of this is added VAT at the current rate of 17.5 % - there is no luxury goods tax or anything just plain old VAT at its standard rate.
If your order contains books which of course don't warrant any VAT in the UK, then ask the supplier to mark the books and their value on the customs declaration form so that VAT isn't charge don their portion of the order. Its a devil of a job proving to them after the event that the parcel contained books so that you can get a refund. I've only managed to get this refunded with some difficulty, their policy seems to be if it isn't declared as books, tough. Which I suppose is understandable.
Then the Royal Mail charge around £7.95 to collect the charges mentioned above. This is sometimes the killer as this amount can really narrow the savings made.
I tend to work on a value of 22% being added to the total cost if I'm caught by customs. I've been purchasing stuff over the internet for the last 6 years and now I have probably 1 in 3 parcels charged compared to nearer 1 in 10 a few years ago. I expect that HM Customs have been given more resources as internet purchasing has gone up to catch us bargain hunters.
I hope that helps make it a bit clearer