UNREASONABLE ADMIN FEE

In recent months I signed up with Vox Telecom for their fibre service. Everything was okay until this month when there was insufficient funds in the business account and the debit order bounced. Yes, I was remiss and I accept responsibility. The same day as soon as I received the email informing me of this fact I settled the account in full. Now Vox Telecom are threatening to suspend their service as I have not paid an admin penalty. Does sending a reminder an email seriously warrant a penalty and are they seriously willing to lose me as a client over a lousy R35 ? This attitude will not win them many new clients.