furious at delay of an overseas package

on the 28th of December 2017, I placed an order with a London supplier (5 shirts), and they sent it via Royal Mail to South Africa. the tracking number states that it arrived at Johannesburg customs on the 30th December 2017, and since then I have had no word about the package. It is now the 22nd of February, and the grace period for patience and courtesy is well past and over.

please furnish me with details on how I can follow up on my package? emailing doesn't yield any response, and phoning just results in being put on hold for eternity.

failing that, please point me in the direction of a means of prosecution. I am beyond furious on how poor this service has been.