Although I agree in principle with this young lady, I'd like to add a little something:
The payment isn't made until the transaction is completed.
Finance companies do not run the post office. The post office is usually very sufficient, but errors can be made. The customer is supposed to make the payment (post it to the account) "on or before the due date". Several days after the due date, a late fee is typically imposed. That fee is stipulated in the contract. Some states put a cap on the late fee, others don't allow them at all.
If you mail a payment out, and it hasn't cleared your bank, it might be lost in the mail. No, it is not the customer's fault. By that same token, it is not the fault of the finance company. However, it is still the customer's responsibility to make the payment (post it to the account).
Look at it this way: Your company mails your paycheck to you. If you don't receive it, whose responsibility is it? You letting your company know you haven't receive it is the same as the finance company notifying you. Your company reissuing a check is the same as you reissuing the payment. In principle: no difference.