Ruthless FNB

FNB decides to put through a credit card collection on the 25th of the month which is NOT what I initially set up on my account. So the payment went through as unpaid. Then they send an email asking me to pay DOUBLE the installment and then they go ahead and debit my account for DOUBLE the installment. I want an answer as why you deducted double the amount of my minimum payment. This is an unprofessional bank and absolutely ruthless.