Custom charge for membership?

[Kropotkin]

Couple of week ago I have decided to join a certain *activist* organization operating in the US (I am the UK resident).
The price of membership was $25 (roughly £20-21) and as a member I am eligible to get Membership ID and a certificate. I have paid the price and the parcel with these items was sent to my UK’s address.

For some reason, my parcel had been a subject of interest of Border Force on behalf of H&M Revenue & Custom officers, who decided I am supposed to pay custom charge (£5.83 + £8 of holding fee by Royal Mail). They indicated this is due to the Notice 143.

I do not understand what is the reason of such charge, while I would consider this parcel rather as a letter than a goods (not even as a gift), especially that both documents included are highly personalised and cannot be for me a subject of commerical use.

I am profoundly curious whether there is any way of appealling of this decision.
Can you advise me anything, please.

[pawncob]

https://www.hotukdeals.com/discussions/how-do-i-dispute-a-royal-mail-customs-charge-1826885