U.S. Bancorp Credit Card Complaint

U.S. Bancorp Credit card department,

Dear Sir, My self - -, using - Credit Card since 2007. I observed in last month that company has practice to charge interest on due amount without adjusting partial payment made on or before due date. On asking clarification on this they told that until unless full amount is not paid interest shall be charged without adjusting any partial payment. Kindly look into that matter and justify why the partial payment should not be adjusted in principal amount for calculating interest.