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@taewoonglee I suggest you contact Optus Billing to sort this out. The Optus Employees in Yes Crowd do not have access to your account details etc.
Sounds like you've received a pro-rated final bill.
You wouldn't be charged for days of directly using the service, you'd be charged for the amount of days you had the plan active for.
For example if you were on a $35 plan and your final bill was from around January 4-18, you'd be charged about $17 even if you only used it for one day on January 10th.