Your platform should allow you to display the currency in the country of origin's currency. Once they checkout your merchant will then process the transaction in your local currency and charge them the converted rate accordingly.
Note there's a technical diffs between Zero-rated and Outside Scope of VAT. Typically sales to foreign entities in a foreign currency are outside scope of VAT. Your tax invoice must reflect the foreign entity's foreign address, ie it must be a jurisdiction outside of scope of SA VAT. Zero-rated technically means that the item (good/service) is VATable but the current rate of VAT is zero. Yes it amounts to the same thing, but it has different legal and accounting implications.
Some SADC-based entities prefer an SA tax invoice as they reclaim SA VAT via their own preferred route.