Milestone Credit Card unethical late fee. Beaverton Oregon

I applied for and received a Milestone MasterCard in October 2017. I registered online immediately and setup auto-pay on 10/17/17 and received email confirmation. The Auto-Pay Start Date was set to 11/10/2017. The first statement due date was 11/10/17 and the payment was posted on 11/10/17. The second statement was due on 12/10/17. Just by chance I checked my account online on 12/11/17. The payment due on 12/10/17 had not been posted. I assumed that it didn't show yet since 12/10/17 landed on a Sunday. I was wrong. I received the current statement email notification on 12/12/17 and went back online to view it. Milestone has charged me a late fee because the due date landed on Sunday. The 'Transaction Date' for the payment due on 12/10/17 shows on my transaction history but the 'Post Date' for the payment shows 12/12/17. It then shows a 'Late Payment Charge' for $27.00' on 12/11/17 and of course that transaction posted on 12/11/17. I did call the company and got the late fee waived but I explained to the customer service rep that this is totally unethical and I've never had this happen before with any other credit card I've setup Auto-Pay on. This is whole reason I use 'Auto-Pay' with credit cards - to ensure I don't forget to pay on time and get a late fee. I was polite but told the customer service rep that this is just wrong. I know it's not his fault. I just want others that have this credit card to make sure you make your payment on a weekday even if you have Auto-Pay setup.