Import duties vary but most clothing attracts duty at 12%. However the UK is currently imposing a trading sanction against the USA (because the USA is breaking World Trade Organisation rules by illegally subsiding their cotton farmers). That means that cotton-based goods, from the USA, currently attract duty at 27%. (Duty is ignored, though, if it comes to less than �7).
Then there's VAT which is charged, at 15% on both the value of the goods and on top of the duty. (VAT is waived if the value of the goods is less than �18)
Lastly, there's an �8 inspection fee to be paid for examining your goods. (If there's no duty or VAT to be paid, the fee is waived).
Examples. (All are for cotton-based clothing):
1. You buy an item for the equivalent of �15. The duty is theoretically �4.05 but it's waived because it's less than �7. The item is worth less than �18, so there's no VAT to pay. Since there's no duty or VAT, the inspection fee is waived. Amount to be paid to postman, upon delivery = Nil.
2. You buy an item for the equivalent of �20. The duty is theoretically �5.40 but it's waived because it's less than �7. However the item is worth more than �18, so you pay 15% VAT = �3. There's also an �8 inspection fee. Amount to be paid to postman = �11.
3. You buy an item for the equivalent of �30. The duty is �8.10. You're charged VAT on �38.10 (the cost of the item plus the duty) = �5.72. There's an �8 inspection fee. Amount to be paid to the postman = �21.82.
Chris