While South Africans can have PayPal accounts to make payments from, it is not possible to receive payments to a South African PayPal account. This is why the South African currency (ZAR) is not available on the PayPal drop down list. Only countries that PayPal fully supports are included.
The reason that PayPal payments are blocked to South Africans is because of restrictions imposed by SARS.
In the meantime, you could try out Payfast, a South African alternative to PayPal. You could integrate their shopping cart into your site by copying the embed code they provide. Then, in the Yola Sitebuilder, add the code by following the steps on this link.