My grandma wants me to send her a water dispenser from China to South Africa when l go to visit.

You can take it as a checked luggage. But you should pay for the extra space or weight. Take China Airline as an example, if one checked luggage is larger than 158cm in dimension or weighs over 23kg, you should pay CNY200~400 for that.
You should also notice that when you get back to your country, a declaration and tax are also needed. For your reference, in PRC, the tax rate of a water dispenser is 30%.