My limited experience in the complicated areas of import duty/vat/cost/ carriage etc on purchases from outside the UK suggest to me that there are several inaccuracies in the examples quoted by the site. The first glaring omission is that the site does not quote what the carrier will charge the vendor, you and me, for clearing the goods through H M Revenue and Customs and paying the charges on our behalf. I examined several examples on their site of transactions from the USA and found inconsistencies eg not all transactions showed duty of 3.7% , although all goods from the USA will be subject to this charge.
As a rule of thumb add 30% to the value, incl delivery , insurance etc of all purchases from the USA plus the couriers charges.