Yui Joe Cheung

I'm buying something from China through Aliexpress, the original price is around 250 pounds, but it's discounted to around 130. Delivery is free. At checkout, it says Aliexpress is required by law to collect the VAT, which is perfectly fine. I'm just wondering if would i need to pay the customs duty since before the VAT i only paid £135. Or does the customs duty add the VAT at checkout with the value of the product.