Customs’ Duty Puzzle

Background to the question: as part of my personal clearing up after myself, I decided to offer my Dad’s war time (WWII) medals and certificates to the Buckie & District Fishing Heritage Museum.
The museum agreed to accept them.
On October 3 we took the package to the UPS store and arranged for expedited shipping.
The shipping costs, including the packaging and insurance were $99.91. This included tracking the package as it was scanned at each UPS depot.
We laughed at the notation that delivery was to a remote area. Buckie is not remote!
On October 10 UPS arrived in Buckie, with the package, plus a request for 20 pounds and 59 pence for freight charges.
Freight charges from where? We had already paid for door to door delivery.
I called UPS customer service. They say charges are from UK Custom services.
Why? The medals are of no commercial value and will not be resold. They were a donation.
Of course, I will be contacting UK customs & excise, but if anyone who reads this and who knows about current custom procedures, I’d appreciate information.
I’m going to bed humming, “The deil’s awa wi the exciseman.”