If you see an error similar to:
“Package weight 10kg exceeds maximum weight 5kg for Flat Rate Packages (FLAT_RATE_BOX_MEDIUM).”
This means your order weight exceeds 5kg, which is the maximum allowable weight for Flat Rate Packaging.
You need to change the package type to your own Packaging, or reduce the order weight to 5kg.
NOTE: If you reduce the parcel weight to 5kg in order to utilise Flat Rate Packaging prices, but the parcel weighs more than 5kg you will receive an Underpaid Postage bill from Australia Post - and potentially an additional charge.