better off pricing online, calling airlines direct, and then booking your best rate!
I ususally check expedia, travelocity, major airline websites to price compare. Then I call major us airline to see if they can match expedia/travelocity price (gets around paying the expedia/travelocity booking fee) Us Airways wouldn't do this our last flight, Delta however would honor the lower price. Also Us Airways tickets were cheaper online, but when you call them the price was over 100 more yet they wont honor the web price over the phone...
And i believe you need a credit or debit card to purchase online or over the phone... Maybe you could use check by phone also (not sure if you can though... i have never attempted it) Maybe someone else could add some info in regards to that.