Capital One Credit Card Complaint

Capital One Credit card department,

Payoff process California

I called my credit card co. to pay off my debt in full on -/-/-l. I did, and then I received a refund from an online company that owed me $58.00 and it posted to my credit card on -/-/- as well ( it shows on the statement ). On my next bill, which I got today -/-/15, that refund was absorbed into some make believe charges that the credit card - said I owed in interest, and that I still owed them an addtional $4.00 and some change. I said why was n't that interest included in my payment when I paid off the credit card in full? They said it was to accrue over the entire month when my bill was actually due. Now mind you, my billing date is the - of every month and I paid in full on -/-/-. So, I was charged an entire month for interest on a bill I paid on the first, but they said it did n't matter and calculated what would accrue over the entire month up until -/-/-. That seems pretty unfair. How is it collecting interest if it was paid off on the - of the month 3 days after my last bill was due and paid for before -/-/-, which I paid on -/-/-, and it was more than the minimum amount due. So, I was charged about $65.00 in additional charges after I paid my account completely off. I want someone else to explain this to me, or I am going to think I was penalized for paying my account in full.