Can you tell my what is happening with my bill mow? I recently Made some changes.
I used to have 18mg but I reduced for 10, and should my bill be $75 but I recived bill $133.
What does the bill say? There should be a list of costings.
Your bill might look messy as Optus credit the unused amount of the 18Gb plan and Charge for the 10Gb plan.
However you can't generally decrease your data without a penalty. Depending how far into your plan you are there is likely a $10-$200 fee for switching.
You should contact LIVECHAT here online and ask as they can tell you exactly what makes up your bill and if a fee was applied.