Beware of hidden shipping costs. You may have to hand over another £8 handling fee to the Royal Mail if you purchase goods from outside of the EU. You may not find out about this until you receive a card through the door.
If you buy good from abroad, outside of the EU, and it costs more than £18 it could be subject to VAT. If it is, HM Customs hands it over to the Royal Mail to collect. For this "service" Royal Mail will charge YOU a further £8.00 International Handling fee. So, if the goods are declared to be worth £18.01 it will actually cost you another £2.70 in VAT (at 15%) plus £8.00 handling fee (£18.01 + £2.70 + £8.00) making £28.71. (And you probably had to pay extra for P&P on top of all that). This is legal by the way and the Post Office will not deliver the goods until you pay their handling fee.
Please see "Receiving gifts or buying goods from overseas" at the Royal Mail site for more details.
Please see the HM Revenue & Customs site for more details. They have a guide called "A guide to international post users". It explains some of this in section 3.5. Interestingly enough section 3.4 explains a way of not having to pay this £8 handling fee. It involves the seller being authorised for "Import VAT pre-payment". They can collect the VAT on behalf of UK Customs when you buy the goods and then it doesn't go through Royal Mail International handling.