I think Ethel's estimate of 40% add-on is too low.
Let's start with her figure of �100 for the purchase price and add a tenner on for the carriage costs.
The first thing HMRC will do is to calculate the Duty payable. On most clothing this is normally 12%. However, because the USA is currently breaching World Trade Organisation rules on subsidies to their cotton farmers, the UK is currently applying a trade sanction by adding on a further 15% 'ad valorem' tax. So the duty will be �27.
The next thing to add on is VAT. VAT is charged on the cost of the goods, on the carriage and on the duty. (Yes, that does mean you have to pay tax on a tax!). So thats 17.5% of �137 to be paid. That comes to roughly �24.
There's also a fee to be paid for the examination of your goods. This is a flat fee of �8.
As stated in your post, FedEx charge roughly �4 for doing the paperwork.
So, your total bill, upon delivery, will be approximately �63.
Chris