AusPost may charge you for underpaid postage if the weight or dimensions specified for the order are understated.

If you have unpaid fees that are overdue then an error may appear when trying to generate new labels. You will need to log in to your MyPost Business account and pay for fees before you can continue printing labels.

For any labels that you print using 'Own Packaging' you must declare accurate dimensions and weight, as per Auspost requirements.

Note: If you select a default Australia Post Package for an order and edit the values of the dimensions, it will change Packaging type to Own Packaging.

You will need to check that they were properly declared for those parcels. If you believe they were correctly stated you'll need to contact AusPost MyPost Business support directly and ask that they investigate.
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For how Auspost calculates cubic weight, see:
​https://auspost.com.au/business/business-ideas/ecommerce-jargon-busters/what-is-cubic-weight