This happens in the USA as well since my mom was sick a while back so asked my Aunt to post a package to me from the USA and the UK and they shipped it via a more expensive option than it should have.
I wish someone would have helped my Aunt out since the price she paid was over $40 US dollars while here is an example of costs from the US Mint web site says.
International Shipping -
You will be charged $17.90 plus a $2.95 handling fee per item for the United States Mint's International Shipping option.
Packages shipped using the United States Mint's International Shipping option usually arrive within 1 to 2 weeks.
https://catalog.usmint.gov/customer-service/shipping.html
The other issue is that my Aunt did not know what to put on the customs form.....so the person helped her out. Of course, the package ended up not even leaving the USA since they put coins on the customs form.
Which according to this is a prohibited item:
Coins; platinum, gold, or silver (manufactured or not); precious stones; jewels; and other valuable articles.
As well as:
Coins; banknotes; currency notes, including paper money; securities of any kind payable to bearer; traveler’s checks; platinum, gold, and silver; precious stones; jewelry; watches; and other valuable articles are prohibited in Priority Mail Express International shipments to the United Kingdom.
https://pe.usps.com/text/Imm/tz_018.htm
When my mom was better she reposted the package with hobby supplies on the customs form for a somewhat lower amount than what my Aunt paid.
What is in the package was a 2023 Uncirculated Coin set that they bought for me from the US Mint, the 2023 P/D Innovation Dollar coins, and the S mint quarters clad - non proof that they got bought from a dealer My Mom and Aunt were trying to helpful since sadly my previous US coin source that I dealt with for over 15 years passed away.
Does the US Mint get some sort of bulk discount deal with who they use to ship packages?
How can the US Mint sent items to the UK (or even the Royal Mint to send items to the USA) if coins are prohibited thru the mail?
Who should my family use to post items to me in the UK in the future if needed?
I know all of this is not directly related but in my Aunt's case, they helped her post something for a sum of money can be quite a sum to a person in their mid 80's and they helped put a description down which flagged it as a prohibited item so the package never left the USA but was returned to her.