UK Royal Mail services has added fees on imported packages that come in from overseas that aren't part of the European Union. The two main fees involved in these are VAT (value added tax) and the Royal Mail handling fee.
Both VAT and the handling fee are not charged to the seller, but are charged to the customer once the package arrives in the UK, meaning that the seller does not charge the money for these two fees when the items are purchased since it is on the customer to pay after the items have shipped.
The handling fee for all Royal Mail imports is £8.
VAT fees vary depending on the value of the item imported. Any item valued over £15 will be charged a VAT fee, with a specific formula used for any given amount over the £15 threshold.
Also be aware that some packages will be inspected by customs and incur customs fees if they are examined. This customs fee is added to VAT and handling.
For more information read Royal Mail’s page on importing goods and customs.
If you ship internationally, inform UK purchasers that their package is coming from the USA and may incur these fees when it arrives and it is up to them to pay so they can receive final delivery. Royal Mail provides a link on their customs and imports page to make a customs payment online.