Thank you for reaching out, @markcoram. It appears that that the $12.00 Charge is your 11/36 Device Repayment Fee for your Samsung A50 Black. This would be outlined in your contract of your attached mobile service. We have a Support Article that goes more into detail about Device Payment Plans & Credits.
If you would like us to take a closer look at your billing/service feel free to utilise 24/7 Messaging Service via the My Optus App.
