Please be informed that we have processed all your outstanding invoices sum of US $ 15,336.85. We apologize for the irregular delay turn out this time and we guarantee that your future payments will receive in due course.
How about '[This is to let you know that - necessary?] we have processed all your outstanding invoices, which add up to .... We apologize for the delay this time, and guarantee that all your future payments will be promptly dealt with.' Or, you could just replace the last comma with a semi-colon, and end 'it won't happen again'.