A letter of complaint

This was what I sent to my service provider for over-charging and agreeing to waive off the incorrect data charges of close to $300 and payment was made. And after agreeing that no payment needs to be made in the light of the credit balance, I received a reminder letter. So much so for being with this red umbrella for at least 5 years. Peeved. ANd this is the SECOND time in the course of two years I've encountered wrong billing by the red brolly.

Here's what I wrote. (I used up every single letter available on the internet form!!)

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I was served with a reminder letter dated 1 Sept 2009 despite calling to your billing hotline 1688 three times, due to incorrect charge on data usage (the new phone I bought  comes with a free one month unlimited data usage).

Your officer informed that the amount overpaid has been credited back to my account and no late penalty charges will be levied should I opt to offset the August bill of $88.58.

Hence, I was surprised to receive this reminder letter since I'd overpaid and YET was served with an warning letter with potential penalty.

No other form of customer service recovery was offered and except to waive off the excess charge and credit the amount back to the account.

Please advise if I need to proceed with the payment since it's ridiculous that payment has to be made for the bill despite having credit balance in the account.