The way our billing system cycles can be confusing for customers, I understand. But this customer went off the rails midway through the conversation, and I never found her again. We generate and mail out bills on the 1st of each month. We operate under the official assumption that customers will receive their bills by no later than the 15th, and bills are due on the 25th. If we haven't received your payment by the last day of the month, the bill we generate on the 1st of the next month includes a blurb about how that is now considered past due, and we add that amount to your current bill. Thus, if you haven't paid your January bill by the 31st of January, your February bill includes Jan+Feb to become your total.
Now, if we get your payment after we generate your bill, but before you get it, you will get a bill including the amount that you already paid. Enter the customer.
Customer: "I have this bill from you that says I never paid for January, but I mailed out the check on the 20th."
Me: "All right, I do show that we received your payment on January 31st, it looks like our billing system sent out your February bill before that had fully posted, which is why it's showing on your February bill. I do apologize for that, I'll make sure you don't see any late fees, and all you owe is the amount for February, which is $X."
Customer: "But I don't understand, why would you add that amount to my February bill when I paid it?"
I explain the above information to the customer, apologize again, and tell them that it took about 24 hours for the payment to get from our mailroom to our billing system, by which point the new bill was generated.
Customer: "But I mailed that payment on the 20th! Why did it take so long for you to put it in the system?"
Me: "Well, that's something that you would need to take up with the Postal Service. If you mailed it on the 20th and it took 11 days to get to us, then the delay happened with them."
Customer: "But I don't understand why it wouldn't be in your system until the 31st."
Me: "...Well, because the post office took that long to get the check to us."
Customer: "But I mailed it on the 20th! Why don't you have it in your system that I paid on the 20th?"
Me: "...Because we didn't get it on the 20th. We got it on the 31st."
Customer: "But your system should list it for the day that I mailed it!"
Me: "I definitely see where you're coming from, but unfortunately, we can't list the payment in our system until we get it. I get that once you've mailed it, you consider it paid, but we don't consider it paid until we actually get the check."
Customer: "I don't understand this. It doesn't make any sense. You should accept the payment for the day I paid it." Click.
I'm condensing slightly, we actually went around about 3 times before the customer gave up.