Buying plane tickets at the airport is only cheaper for that charge specific "online booking" or "carrier interface" fees . For major airlines like Delta, United, or American, buying at the counter is almost never cheaper and can sometimes be more expensive due to agent service fees. How the Savings Work
Budget airlines often include a fee in their online price that can be legally bypassed if the ticket is purchased in person. This is often a tax-avoidance strategy for the airline, as they don't have to pay a 7.5% federal excise tax on "optional" fees like these. Is Buying Tickets at the Airport Really Cheaper? cheaper to buy plane tickets at airport