In the UK, the threshold for tax being due on items from outside the EU is currently £15. Upwards from there, you are liable to 20% VAT, which is sort of OK. However, Royal Mail will also charge you a fee for handling the payment on your behalf, this is a flat rate fee of £8. Thus, an item worth £14.99 will arrive 'free' after you pay the vendor a total of £14.99 for it. An item judged to be worth £15 on arrival will cost you a total of £26 ( 26 = 15 + 3 + 8 ) - and be subject to a sometimes considerable delay.
Buying an item close to the threshold has an element of risk - the 'value on arrival' is not within your control, it is down to an assessment made on 'your behalf' at the time of arrival - you may even have a receipt for an actual payment below £15, but a further slide of the pound during transit can tip you over into being liable to the Royal Mail ransom + VAT.
Paying for something with 'free postage' can add to your woes, if the vendor then declares the total to be the value of the item - much better to have the item value and postage as separate amounts.
The threshold issue may not be such a problem in the model railway world, but it does cause some trepidation around CDs, DVDs, etc.