Dang credit cards

We had a credit card balance that we've been paying off using an online autopay on the same date every month (which has been the earliest the payment was due). But they changed the due date without warning to five days earlier. Now we have a $39 late fee charge for this month & they jacked our interest rate up to 19.99%! I'm sure I will be on the phone all week trying to straighten this out with their "customer service" department.