I finally found where I can buy the prepaid bus card in Anjeong-Ri. It's called the U Pass and you can use it to pay for the city bus(the 20 bus goes thru Anjeong-Ri) and also for the metro. There's also a card called a T card that is the same thing just from a different company. The card cost 1.50 and you put however much money you want to on the card. The card is rechargeable. When you board a bus or subway/train all you do is swipe the card and the fare is deducted from the card. You get a small discount when you use the card also. To get one of these cards; go down the main shopping mall road that is in front of the walk-thru gate, when the road changes from a one way street to a two way street you will see the Paris Baguette bakery on the left hand side. There is a small store beside the bakery that has a green lotto sign. This small store is where you get the U Pass. I love this card. I've used it a lot on my trips to Incheon, Suwon, and Seoul. You can also get the U Pass at a small street stand just in front of the Pyeongtaek metro station.

Good to Know!I Neglected to mention the refillable cards on the forum. They work for most all buses and the Subway and trains.
I got mine from the Subway Ticket window. It is called Korea Smart Card, and the only English on it is "T money". After you learn how to ride the train/subway it is a great convenience! And saves 100 won each use. Just ask for "CARD-DU" if they don't speak English, and give them 10.000 won to start. They will credit the card, and you are on your way.
This is very important:
SUBWAY:
When you figure out how to tell directions and use the NUMBERS on the stations, then start your adventure. Go with a Korean the first few times.
Place the card on the RIGHT side of the turnstile with a BLUE LIGHT.
If magnetic strip is DOWN it will deduct 800 won. When you get to your destination, do the same upon EXIT only. The Screen will show what you have remaining on your card. If you go from Songtan to Yongsan, for instance, it will deduct ANOTHER 800 won also, because of the distance involved.
(But you just saved a LOT over the Post /Base Bus fare, as you only paid a total of 1600 won for the one way trip!)

BUS: Enter the BUS from the front, SWIPE YOUR CARD ***Don't FORGET to EXIT from the rear, and SWIPE YOUR CARD AGAIN!!! If you get off and don't swipe your card again, Bad things can happen. I don't know what- but I am told that your card may be charged Double! I really don't know if that occurs the same day or not. There is a time limit, and you can transfer buses freely, within that hour or so, and not be charged of you use the card on ENTRY and EXIT.
If someone can explain it better, please do, and copy to the Yongsan Forum also! Thanks.