I gave a detailed explanation of the rules recently in my post here:
http://www.theanswerbank.co.uk/Law/Question1549636.html
At $18 ( = £14.05) the value of the goods is just under the critical £15 mark, so there shouldn't be anything to pay.
However be wary of buying anything from a non-EU country which costs more than £15 because the charges can add up very quickly. (They can be particularly steep on cotton-based goods from the USA because, on top of the 12% Import Duty which applies to such goods from elsewhere outside of the EU, there's a 15% penalty tax to pay. That's charged on cotton goods from the USA because their government is in breach of World Trade Organisation rules through providing state subsidies to US cotton farmers).
As an example, if the dress had cost $25 + $10 carriage ( = £27.31 in total) there would be no Import Duty but there would be £5.46 VAT and an £8 customs examination fee to pay, making the total charges come to £13.46.
If the dress had cost $200 +$10 carriage (= £156.06 + £7.80 ) there would be 27% of £156.06 (= £42.14) Import Duty to pay, plus 20% of {£156.06 + £7.80 + £42.14} as VAT (= £41.20), plus an £8 examination fee, making a total of £91.34 in charges.