Barclay Bank Delaware And Juniper Bank Juniper Bank how not to treat customers Wilmington, Delaware

I have had a Barclaycard/Juniper Mastercard account for 5 years without one late payment.  Two months ago, I mislaid the bill, and found it a few days past the due date.  I mailed it in anyway and next statement saw a $45 late fee on my bill, which I paid.  A few weeks later I got a letter from Barclay saying that my late payment triggered the penalty interest rate of 30%.  I called the number and explained what happened.  They were unwilling to remove the late fee or to lower the interest rate, so I closed my account.