Any route taxi that doesn't have a passenger already on board is a potential "charter" and some will "charter" after leaving off the passenger.
For the most part, they don't cruise around looking for charters but will rarely turn one down as beating the heck out of your car to go back and forth a bazillion times is pointless if you can make a day's wages or more with one comfortable trip.
There are some "recommended" prices for charters to certain destinations but feel free to barter for a good price. Nothing wrong with saving a buck or two and if it is too likkle I am sure the taxi driver will let you know and offer a higher price.
Peace and Guidance