Namibia along with Lesotho, South Africa and Swaziland is a member of the Southern African Common Monetary Area and as such the Namibian Dollar NAD is pegged 1:1 to the South African Rand ZAR. Both the Namibian Dollar and South African Rand is legal tender in Namibia though change will usually be given in Namibian Dollars.
Banks in Namibia will convert Namibian Dollars for South African Rand and vice versa without charge or paperwork. Since any bank or currency exchange outside Namibia including other members of the Common Monetary Area will charge a substantial service fee to change currency, it is advisable to make use of a Namibian Bank before leaving the country.
It is also advisable to carry proof for example, ATM receipts that money you are taking out of the country is money that you brought into the country in the first place.
Automated teller machines are available in Windhoek, Swakopmund, Luderitz, Tsumeb, and other towns and cities. It is best to use only teller machines that are inside a mall or other building. Many machines have guards in the larger centres. Always be careful to make sure no one is watching you enter your PIN, and be vigilant about typical scams e.g. machines that seem to eat your card and won't give it back after you enter the PIN.
Credit and debit card fraud is a major problem in the country. Make sure you get a receipt for all processed and cancelled transactions. When entering your pin for a purchase, it is often customary for clerks to navigate past the sale price confirmation screen on POS terminals by clicking "ok" to the total sale amount on your behalf. While not a best practice for electronic financial transactions, it's *usually* not an ill intended maneuver on the part of the cashier. If you remain aware while completing transactions, you shouldn't have any problems.
Prices in shops are fixed, but prices in open markets or from street vendors are open to barter.
In most towns you will be approached by many locals to buy souvenirs, when this happens a 'no thanks' will usually suffice and they will leave you alone. It is common to haggle. Try to buy as much as possible from small shops instead of bigger ones -- it's the best way to help the poor local population.
The cross-border money transfer facilities are limited and expensive, with one of the poorest currency buying-and-selling rates, because government does not want the money to be sent out of the country. There's a western union office across the street from the US embassy in Windhoek.