Truckers who shopped at Wal-Mart March 31 need to look at their credit card bills and make sure they were correctly charged.

The company that handles credit card transactions for the retail giant, First Data Corp. of Denver, said in a release that shopper transactions posted April 1 – which would be for items purchased March 31 – could have been double or triple charged due to a problem with the company’s software.

The Associated Press reported that the problem could affect more than 800,000 transactions where a Mastercard or Visa was used.