Probably a scam or an error in the system. Might pay to have a look through the my account portal and see if you notice anything strange, ordered or have a word with support on the live chat link below. You just never know these days with all the fraud going on.
Its likely not a scam (likely) but just poor programing and policy by Optus.
Have you used the MYACCOUNT online or Optus app lately?
Various insignificant changes in there (like a new Nickname for a bill) will result in "Order Confirmation" emails.