When a merchant in the U.S. sells to a customer in the United Kingdom, the UK mail system will hold the package until Value Added Tax (VAT) and an international handling fee are paid.
Unfortunately, USPS does not currently support pre-payment by the merchant for these fees.
If your customer has not received this notification, they will need to contact the Royal Mail service directly.
Starting January 1st, 2021, all goods shipped to the UK are subject to VAT regardless of the price of the goods in the shipment. Previously, the threshold was only goods over £15 were subjected to a 20% VAT. You can use the Avalara VAT Risk Assessment tool to find out if your company will need to be VAT registered.