Product: Credit card

I made a purchase and was approved for a store credit card at XXXX. Amount XXXX on XX/XX/XXXX. I was immediately billed and payment was due on XX/XX/XXXX. In my mind, this is a completely scandalous thing to have a bill due before 30 days. I then see that the interest rate will be 35.24 % which is the most I have ever seen, and is very predatory! 
I have plans to return some items, holidays get in the way, I return the items XX/XX/XXXX and then plan to pay my bill. That same day I activate the card. I returned {$200.00} of my {$340.00} purchase. And I should have paid the remainder of my bill in store, but didnt think it would be a problem to set up online and pay that way. I go home, and can not setup my account online. Nothing works and they say they dont have my info correct. 
I am finally able to call on XXXX and speak to someone in XXXX. He said they dont have my email or phone and that is why I cant setup my account. I have been receiving emails from this company all month. And when I called he read me my phone number as part of the verification. 
I complied and gave my email and phone. He sent links to my email and verified it, then he sent a link for a direct login to the account. I hang up, seeing that I can log in, I get to another page and they do not have my email or phone, so I can not login and can not pay my bill. 
I figure, maybe it is a slow system, I wait a little while, and can still not login. I then decide, I must make my payment, so I call to make a payment. The phone system tells me it will be late, because I am calling after XXXX XXXX XXXX XXXX. Which in my mind is designed to XXXX me over. I have never seen this early of a cutoff time, and I am on the XXXX XXXX. Predatory! 
There is another option to pay {$9.00} for expedited payment. I push that through and ask the next XXXX representative if my payment will be made today. He says yes. I say great charge me {$9.00} so that I can be relieved of this Mess. I offered my checking info and a payment of my remaining balance, XXXX. And when he confirmed it, he said that it was going to be made today XX/XX/XXXX which is XXXX  for me, and Also late. I tell him that I was encouraged to make this payment because it would be made XXXX. He acts highly confused and acts like he has no answer or fix to my issue, he is just doing his job. I ask for management, he got scared and tried to confirm all info again. I tell him, this is not about him, it is about this scandalous company and how they are effectively XXXX  me over. I feel like I am getting nowhere, so I tell him to process the payment, so that maybe I can talk to someone during their business hours XXXX. 
He said he processed it. I was so frustrated and upset, I just wanted to wait for my bill, to see the result. I figured they wont charge me interest because it will be paid and maybe not a late fee, because of XXXX  XXXX  from my return in store. No payment came out of my account, and today I call their automated system, because they still have not let me into my account online. I have been charged {$40.00} interest and my payment was never made. I try to call them and reach another person in XXXX, even during normal business hours. And she says she does not have any record of my payment. I have no words, except for bad ones, which is unproductive, so I hang up after saying how upset I am yet again. It is my understanding that this company set up every step of this process to confuse me and charge me interest. No one is taking responsibility for these obscene credit practices and I believe they should be investigated.