Incorrect calculation of duties and horrible customer service

We received a package from my mother-in-law as a gift for our children. The value of the goods is R700, she paid R1600 for TNT to ship the goods and now TNT is requesting an additional R400, allegedly for customs duties.

In South Africa, gifts have an exemption up to R400, so duties should be paid on R300 only - that's if I don't go into the fact that there are 3 ultimate beneficiaries of the gifts, which should bring up the total to R1200, but let's not get complicated.

Any person in their sane mind cannot think to charge R400 duties on a R300 value. Several calls to try and resolve this have gone unanswered and usually end up with a "number has been disconnected" message. Emails go unanswered and goods still haven't been delivered due to this outstanding payment.