McGraw Hill, which controls 21% of the higher education market, reported in March that its digital content sales exceeded print sales for the first time in 2015.
The codes—which typically range in price from $80 to $155 per course—give students online access to systems developed by education companies like McGraw Hill and Pearson.
McGraw Hill didn't respond to a request for comment, but its CEO David Levin told the Financial Times in August that "in higher education, the era of the printed textbook is now over.