Customs tax on a posted letter.

I happen to have had a greetings card and a club membership card ( credit card size) posted to me from the UK only to find when I collected this at the Post Office nearby that a a Customs duty / Tax of R 39.00 was payable . This is really frightening as this posted item has a value less that GBP 0.22 . How on earth can R39.00 worth of Customs Tax and duty be derived from this.