Ordinarily one would need an overseas bank account, especially to pay for a policy that was taken out whilst travelling abroad. International bank accounts come with their own set of challenges (which I will discuss in other posts).
Ideally the most convenient way will be a debit order from your South African bank account even if the premium is USD denominated. Remember as a South African you have annual allowances that are free from exchange controls subject to certain conditions: currently ZAR 1 Million single discretionary allowance and ZAR 10 Million foreign capital allowance. The annual ZAR 1 Million single discretionary allowance will be the most convenient to use.
Imagine if you could fix the exchange rate for several years.