You Owe Me An Apology

In Nov 09, I ordered books for office use. After delivery, I handed the invoice over to the processing clerk and assumed she would follow up with payment. Fast forward to Jan 2010, the company called me and expressed concern that they had not received the payment for that order. Immediately I apologised (thinking my clerk must have overlooked the payment), and assured them I would look into the issue. I immediately went to check with my clerk, who reverted soon after to confirm we had made payment. Hence I informed the other party accordingly, with some payment details. Subsequently they called me back to explain that they did receive the amount, but because there was no payment advice attached, so they were not able to match the amount with the invoice, and hence the invoice was still considered outstanding. Throughout this conversation, there was NO apology at all from the other party. It was as though the whole thing was not their fault, and it was because there was no payment advice from us hence they overlooked . So it means, its still my fault that you got paid, but u thought we din pay because we din tell u we paid u? F* la. You can use common sense right? My order amount was $1353.55. How high are the chances that you have other orders with this exact amount during this period?