There are two Hong Kong travel websites which allow you to buy Hong Kong flights online with instant confirmation. One is www.zuji.com.hk, which I have been using since its debut a few years ago. Most recently, another website joined the ranks, www.webjet.com.hk. The website’s search results give you a glance of airlines, their ticket prices as well as the flight duration and time. The results are based on a single flight with a handling fee of HK$50. I have not used its service so I cannot comment.
Zuji is a bit different. The booking is based on return flights. No handling fee is charged. My experience with it has been good. Its flight ticket prices are among the city’s lowest. But it comes with one disadvantage. If you want to change your outbound flight time after the e-ticket is issued, you have to pay a charge, a practice that you won’t encounter if you buy your flight ticket through an “offline” travel agency.