In general, you have to pay import charges (for internet purchases from non-EU countries) if the value of the goods exceed £18.
The following is based upon $130 being roughly £80, and estimating carriage costs at £10.
Step 1:
Calculate the Import Duty.
For clothing that's normally 12% but the USA is in breach of World Trade Organisations through providing state subsidies to the US cotton industry (giving that industry an unfair advantage), so the UK imposes an additional 15% surcharge on cotton-based clothing from the USA. So the duty is 27% of £80 = £21.60
Step 2:
Calculate the VAT.
That's charged on the value of the dress, on the carriage charges, and on the Import Duty. (Yes, that really is a tax on a tax!). So you've got to pay 20% of (£80 + £10 + £21.60) = 20% of £111.60 = £22.32
Step 3:
Include the examination fee.
To add insult to injury, Parcelforce Worldwide (who carry out routine customs examinations on behalf of HMRC) are allowed to charge you £8 for the 'privilege' of having them calculate your bill.
Step 4:
Add it all up:
£21.60 + £22.32 + £8 = approx £52.
So, in total, your dress would cost you £80 + (say) £10 carriage + (an estimated) £52 in charges = approx £142.
Chris