A word of caution when buying from outside the EU:
If the goods are valued at more than Â£15 (calculated using the monthly exchange rate published on the HMRC web site), there will be additional fees to pay. Royal Mail charges an Â£8 handling fee and you will be charged VAT at 20% (plus any applicable import duties).
The seller has a legal obligation (under international postal regulations) to record the correct value of the goods on the customs declaration. And customs have the right to open any package to check.
In my experience they are very strict. I have been charged VAT on goods purchased from the USA which exceeded the Â£15 limit by as little as 12p, according to HMRC's exchange rate.
And there can be a significant difference between the HMRC exchange rate (set in advance at the start of the month) and the current market rate. On that occasion the actual amount I paid with my debit card (converted from USD at my bank's daily rate) was less than Â£15.