Paid up letter

Called Tenacity on the 08th of November to obtain a settlement quote for my Ackermans account. Agent gave me the quote telephonically, didn't advise payment should be made before the 11th. Settlement was paid on the 10th of November, proof of payment sent on the same day. I call on Monday to ask them to issue me with a paid up letter only to be told I still owe R240.00 because even though payment was made on the 10th, reflected in their account on the 11th and only allocated to my account on the 13th, their book closed on the 11th. So I am liable for the balance. Which I find extremely unfair especially as I was not told to make payment before the 11th. Still waiting for feedback regarding my complaint.