Customs Duty on cotton-based clothing is 12%, so that's £10.80 to start with. (The duty would automatically have been waived if it came to less than £9, so you could have avoided it by splitting your order).
VAT, at 15%, is charged on the cost of the goods, the carriage charges and the customs duty. (Yes, that last bit really is a tax on a tax!). You can only avoid the tax if the cost of the goods is less than £15. So, ignoring the carriage element, that's 15% of £100.80 = £15.12.
So our running total is now £10.80 + £15.12 = £25.12.
Then there's a Customs Examination fee to be added on. (That's only waived when there's neither Customs Duty nor VAT to be paid). In practice, routine examinations are carried out by carriers, acting on behalf of HMRC. I don't know what DHL's fee (agreed with HMRC) is but if the parcel had come through the normal international mail system, Parcelforce Worldwide would have charged £8.00. So the total to be paid should be in the order of £33, suggesting you've been UNDER-charged by a fiver.