Other poster is correct - when you book with one of the Orbitz/Expedia/Priceline type sites it is VERY DIFFICULT to make any changes if you need to. You can use those sites to find specific prices, but I would highly recommend booking directly through the particular airline's website. It'll be the same price. Kayak is the best site for this, no bs, includes most major airlines.
Cheapoair is actually what they call a "consolidator". Means that airlines can sell bulk tickets to these companies wholesale to make some cash on tickets they think they wont be able to sell, then these wholesalers try to move them out at a profit if they can. You can get some good deals here, but they're usually shitty flight times or wierd connections.