Last fall I purchased 3 Spirit tickets at the TPA counter to avoid the usage fee and saved the money for each ticket. So it can be cheaper when you save the fee plus at TPA you get a free hour of parking. The agent said she was surprised the fare was the same at the airport vs what was available online.
She told me generally they try to price the fare higher at the airport to make up the usage fee difference that you would pay online. That wasn't my experience however as I got the lower price, but I'm curious if more try this if that's the case for you.