THEY DONT KNOW HOW THEIR INTEREST CHARGES WORK

Second time complaining about the same issue!

Discovery has NO IDEA how they charge interest on a credit card. No one there can answer the question. I keep getting an interest charge even when paying the FULL closing balance 3 days before due date???? How is this possible? Every month I get this charge. I will be closing this account and queueing at the airport security gate from now on. Shocking service!