Each country have a currency unique to itself.
In Namibia, both South African Rands and Namibian Dollars is accepted as payment for services and goods. These two currencies trade one to one, although it seems like the South African Rand is worth less than the Namibian Dollar, as you pay more in ZAR currency if you pay by card, having a South African bank account.
In actual fact, that is just the forex charging a conversion fee for each rand they convert to Namibian Dollars from South African Rand, besides the international banking service fee being paid when activating international banking on the card. So, it cost more in South African Rand than the actual prices marked on goods and services.
Beware, if you tip on your card when paying for goods or service, you also get charged VAT on the tip, additionally to the VAT you already pay, so the tip is the tip plus VAT. It is better to tip in cash and charge the bill as is to your card.
