JR Bus Kanto

One of traditional bus companies in this area. On Narita to Ginza/Tokyo route, they operate more than 100 services everyday. Their fares are reasonable.
With their service THE ACCESS NARITA, there is no need to buy a ticket in advance. Just line up and pay the driver as you board the bus (cash or IC card).
Although they have an online reservation page, it will be hard to use it because the page has only Japanese contents.

URL : http://accessnarita.jp/en/home/