There are several issues that need addressing.
Primarily, please be aware your company’s tendency to send letters is not fulfilling the legal requirement of serving invoices or notice, and are thus, illegitimate. The primary cause of this is the lack of an avenue of response equal to that of the attempted notice and a lack of an identifying and unique invoice number. If a letter has been posted, it must have a return address. If something is emailed, there must be a return email, if it is a phone message a return number must have a direct number to contact and not an automated system that places people in a que. Failure to provide the same avenue of response as the notice attempted to use is a unilateral communication and is legally invalid.
You may not;
Any action taken under the assumption that illegitimate communication has been legally served will be invalid and the recipient is within their rights to charge a fee to your company, just as you charge late fees.
No invoices are payable until the above has been achieved and charging a 'late fee' for something that hasn't been legally invoiced is tandamount to extortion.
Finally, you have no right to charge anyone 'paper bill fees'. In Australia, our legal system hinges on post. If you charge for sending paper bills (not that they achieve the legal minimums to be considered legal invoices) your client is permitted to charged a "bill payment fee" with a charge far exceeding the costs of the 'paper bill fee'.
Thank you for your feedback.
We will investigate and come back to you with a private message.