BIRMINGHAM, Ala. — In December, they begin showing up in empty storefronts in neighborhoods where empty storefronts are easy to come by. Cars with phone numbers brightly displayed on the doors roll down the streets, and signs pop up along the sidewalks promising fast money.

For millions of low-income Americans, tax season means the biggest one-time influx of money all year. It also means the annual sprouting of commercial tax preparers: some of them big-name franchises, some mom-and-pops and some, as 20-year-old Brittany Dixon discovered this year, shockingly expensive.

Ms. Dixon, a supermarket cashier and college student, took her tax documents — a W-2 form and some education expenses — to the first place she saw, in a storefront near the interstate. The preparation took about a half-hour, and Ms. Dixon was told the amount of her refund — and that she would be charged nearly $400, about a quarter of the total, in fees.

She told the preparer not to file, she said, and found a service willing to do her taxes at no cost. But by then, the first preparer had already filed and taken its cut. “That was my whole car note,” Ms. Dixon said.