There is extremely good news when it comes to refueling in Namibia, some-one else does all the work for you although it is probably worth making sure the petrol attendant uses the correct fuel for your engine! The attendant will also happily wash your windows, check your oil & water and inflate your tires. Generally it is considered good form to tip the attendant before leaving the petrol station.
The network of service stations is very good with most small towns having at least one. Petrol stations will generally have a shop attached (handy tip - these are often the only place to buy off-licence beers on a Sunday).
Credit cards are accepted but it would be advisable to carry cash with you just incase.
We were going to tell you what a litre of fuel costs, but really it changes so often that we could not be bothered, by the time you read this the price will have doubled or halved - depending on the state of the US$. The point is you need petrol to get around and are going to have to pay for it whatever it costs.
A good general rule when you are travelling in more remote areas is to re-fuel when you have the chance. The next petrol station will probably come along way before you need it, but occasionally they may be out of fuel.
"Oil prices have fallen lately. We include this news for the benefit of gas stations, which otherwise wouldn't learn of it for six months." Bill Tammeus