Tariffs on US bound packages came into force on 29th August 2025.  There is now a method for me to pre-pay these charges vis Royal Mail International Tracked Airmail with Postal Deliveries Duties Paid (PDDP).  This has proved to be highly successful with all packets reaching their destination.  The £1.50 administration charge and a set tariff amount has been included in the postage price to US destinations.  While this makes deliveries to the US pricey, it is now possible.