On that note, AmericanExpress always has been my top choice for airline tickets. An account is not required, but by providing your email address, they send you instant updates on flight changes and cancellations, if that happens. They do not charge a convenience fee, and the price displayed is the total you would pay, taxes and fees all-inclusive.
Also, the service is amazing. Last time my flight times changed and my connecting flight left BEFORE I arrived at the connection. I called AmEx, who started to work immediately on the problem. Instead of connecting me directly to Delta, they went on hold for me and called me back after everything was settled. I love the people there. :)
While round-trip tickets appear the most cost-effective, it really depends on the itinerary. For us, we prefer convenience over saving a bit of money. Had we been leaving Europe right after hiking, we would have round-tripped from Geneva. But Spain calls, so we flew out of Barcelona for a bit more money but a nice cap to the journey.