It depends on the value of the goods you are importing. For instance goods above £630 in value are subject to a flat rate of 2.5% customs duty plus VAT on the total value of the goods plus the customs duty (yes - they charge you VAT on the duty!). It can vary depending on trade disagreements between the EU and the US - some products can have restrictions or extra duty depending on political wrangles at the time. They don't usually collect duty if it comes to less than £9.00 but you will still be charged VAT. Sometimes the courier will also charge a fee of £10 or so to pay the duty and VAT on your behalf then collect it from you. That is unavoidable and you will need to pay the amount due in full before they will release and deliver the goods to you. I normally add an extra 25%-30% mentally when buying goods from outwith the EU to cover myself.