U.S. college students, already struggling to pay high tuition and fees, are increasingly seeing a new expense showing up on their bills: textbooks.

It’s becoming more common for colleges to automatically charge students for course materials up-front at the start of the semester, as a result of “inclusive access” agreements with publishers, according to a new report by the U.S. Public Interest Research Group, a non-profit organization.

The practice limits the ability of students to find cheaper ways of getting hold of textbooks, whose prices have risen at roughly double the rate of inflation over the past decade -- like renting or trading them, buying second-hand ones, or accessing cheaper digital editions. Already, students spend more than $3 billion of financial aid a year on textbooks.

“Publishers reap the benefits while students have fewer options than ever to save money,” said Kaitlyn Vitez, higher education campaign director for the U.S. PIRG Education Fund. “These programs create a virtual monopoly, undercutting academic freedom and low-cost options.”

The rules for how students can opt out of the arrangement are often unclear, and it’s likely the automatic charging will increase the price of textbooks rather than deliver a discount, the PIRG said. While the Higher Education Opportunity Act of 2008 mandated that schools and publishers provide details such as the alternate textbook formats available and their prices, the rules haven’t been implemented consistently, it said.

The group published its findings after obtaining automatic billing contracts for 31 public higher education institutions, out of 52 requests filed through open-information laws.

This article was provided by Bloomberg News.