Product: Credit card or prepaid card

This complaint is against Discover credit card. I called in on XX/XX/18 to make sure my payment had posted in the amount of {$82.00} that I made on XX/XX/18 due on XX/XX/18. Part of the payment was a cash rewards credit of {$10.00} I was told by the rep and saw online that the {$71.00} posted but not the {$10.00} in cash rewards as it was still stating {$10.00} was due XX/XX/18. THE CASH REWARDS IS ALWAYS IMMEDIATE IF BEING CREDITED TOWARDS THE BILL! The rep said it would take 3 business days to post. I was extremely disappointed with this deceptive practice as previous cash rewards posted immediately. Had I not checked to see if my payment posted I would have been charged a late fee. I also asked the rep about the interest charge and she assured me that any purchases that I had made within the 14 month 0 % APR period would not be subject to any interest and said that any purchases made after the introductory rate would be subject to interest. I was told that if I dont make any purchases on the card and as long as I continued to make at least the minimum payment required I would not be subject to any additional interest or other fees of any kind. I received a statement online on XX/XX/18 adding interest of {$62.00} and I have not used the card or made any purchases of any kind. On XX/XX/18 i made an additional payment of {$10.00} because I did not trust that cash rewards payment would post on time as was told by the rep. Discover advertised the 14 month 0 % APR but later on without disclosure adds interest to those purchases made within the 0 % APR purchases once the promotion ends. This is fraudulent and deceptive business practices. I am well aware that whatever purchases I make after the introductory rate that those purchases should be subject to the interest rate..but no purchases made within that 14 month period should be subject to the interest rate. Discover interest rate is extremely high. When discover signed me up I explained that I am a XXXX person on XXXX XXXX and my income is fixed and limited. This is elder abuse and they need to be held accountable I have great credit and have never missed a payment nor payed late. Discover stated that I would not have to worry about any interest unless I made purchases then I would be subject to the interest rate. I trusted them to be credible and reputable and they have destroyed my trust in their business to be fair, transparent and customer friendly. I am completely appalled by their deceptive and fraudulent business practices. It is extremely disheartening that I was lied to and taken advantage of. Discover should not take advantage of people with XXXX who are not even capable of defending themselves in such financial predatory situations. I have tried to resolve the issue but they dont care in the least bit. Today XX/XX/18 I closed the account as I will no longer do business with this discover. I will continue to make payments on what I actually owe, but I refuse to pay interest that I was told I would not be subject to. In conclusion I would like that this complaint be escalated to the highest authority and that someone will assist in making sure that I will not be charged these erroneous fees.