Different cards charge different amounts. Usually between 1.5% to 2.75%.
You pay both import duty (3.5%) and then VAT (17.5%) on the whole amount. The post office or courier might also charge you for collecting this tax as well!
Avoid this by using either Play247 (based in Jersey - no import duty) or DVD Box Office (ships via Europe). Or just make sure total cost is less than £18.