Have you ever been stung with charges for ordering items from US sellers? Unfortunately customs charges are part and parcel (hoho) of online shopping, but I've done a little digging to clear up any confusion over what you get charged for and how to avoid some of these costs when you are living in the UK.
Orders with a value of £135 or over will incur import duty, which is a percentage of the value of the product. You can work out what you charge will be using this calculator. Orders under £135 will not incur an import duty charge so if a website offers free shipping to the UK, it's worth splitting large orders into two (or more!) separate purchases if you can. Some items, such as books and computers are duty free - you can look up prospective purchases with the Duty Calculator's lookup tool. It's also worth keeping in mind that import duty is calculated on the total value of the package, so postage costs and insurance will also add to the order value.
VAT is payable on gifts over the value of £35 and other goods (commercial transactions such as online orders) over the value of £15, plus tobacco, alcohol and perfumes of any value. VAT currently stands at 20%, and this is payable on the value of the order plus any import duties you have had to pay.
Sneaky Royal Mail also charge £8 on any orders you receive that incur import duty or VAT and you will have to pay this before they will release your parcel. If the vendor is using another shipping company like DHL or FedEx, make sure to check what their policy is on this.
Just a note: it is illegal to undervalue a package and you could be liable for a fine if you are found out!