Product: Credit card or prepaid card

I sent a bill pay to Comenity Bank for my Wayfair Credit Card on XX/XX/XXXX. My bank XXXX XXXX shows it was delivered on Thursday XX/XX/XXXX. My bill was due Sunday XX/XX/XXXX ( were they even working and collecting payments on Sunday anyway? So how can by due date be on a Sunday? ) My entire balance at the time was XXXX. 

Saturday XX/XX/XXXX, I received a bill for XXXX. This included a XXXX late fee ( which was equal to my entire balance ) and a XXXX finance fee. Thus a bill of XXXX. 

I call and stated there seems to be a problem. I paid my bill off with my last payment and I sent it by bill pay on XX/XX/XXXX. I call several times the on Saturday once before I could validate on my bank account online that the payment was definitely there on the XXXX. In the first call I said this can't be I sent it on XX/XX/XXXX. The person on the end said, well it was due on the XXXX and we got it on the XXXX. I said could it be that you just took too long to post it once it got there? I said with Covid-19 are you are full processing capacity?. And with this Covid-19 you don't things might not be done on time and there is no leniency? You posted on Monday the XXXX when it was due on Sunday and you work and post on Sunday? She said you the late fee is correct.I said this is XXXX   ridiculous ( I admit to the curse word and I was angry as charging a bogus late fee and as much as my previous balance plus a late fee just sounds crazy to me.. ) I said close the account make sure the account is closed. I said it was there in time and I guess and hung up. I then logged on to my back and saw that that XXXX XXXX showed the payment delivered on the XXXX. So I thought wait I have proof that it was there several days before the due date.. So I called back and said I called earlier and after going round and round on the late fee, I checked and my bank it shows it was delivered on XXXX. It got there in plenty of time to post prior to it being late. I sent it on XX/XX/XXXX and my bank says XXXX business days. I said my bank shows it was there before the due date- XXXX. If you processed on Monday the XXXX ( posted ) how was it not there prior to a Monday? She said we processed when we got it and you are due the late charge. I said I sent it in time for it to arrive without being late She said we don't control the mail and it was late. I said how can that late fee be so high? You charge as much as the balance is for the late fee? That doesn't see legal. She said we charge XXXX of any balance higher than XXXX and if it is under we charge the entire balance. I said that seems a bit unethical. You charge a late fee of the entire balance? Wow that doesn't seem right at any rate. I said I have proof from my bank you got it before the due date so there is no justification of the late fee. I said I will send it in so I won't get another late fee and then do some research on my options to keeping fighting this as this is wrong, but the account is closed correct? I won't ever shop at Wayfair again. She said yes, it was closed when you called earlier. Then I hung up. And that is when I found your site. So they can just say I know what your bank says but we didn't get it? That doesn't seem right when I show it got there from my bank on XXXX. That doesn't seem right as a consumer.