Customs Duty on many types of clothing is charged at 12% but it's waived if the value of the item is less than £120 (or if the duty comes to less than £7). So you've got nothing to pay from the first stage of the calculation.
Import VAT is charged at 17.5% (currently temporarily reduced to 15%) on the total of the value of the goods plus the customs duty (where applicable). It's only waived when the value of the goods is less than £18, so you've got about £8 to pay from this part of the calculation.
Routine customs examinations for mail aren't actually carried out by HMRC. Parcelforce Worldwide does the job on behalf of HMRC and charges a handling fee for doing so. The fee is waived when there's neither duty or VAT to pay but (because you'll have to pay VAT) you'll be charged the fee. This is £8 for regular mail services (or £13.50 for certain 'express' services).
So you'll probably have to pay around £16 before the postman will hand over your package.
Chris