The Texas Department of Transportation started mailing $1.5 million in refund checks to some tollway drivers who were overbilled, the Associated Press reported.
About 31,000 drivers who use toll tags already have had credits posted to their accounts after being overcharged.
Officials say software-related problems were discovered last summer when TxDOT changed billing companies.
TxDOT Executive Director Joe Weber said reviews found additional overbillings, AP reported.
The refund checks that were sent, as of March 16, went to customers who paid late fees on bills delayed for months.
Authorities say some billing issues date to 2012.