Australia Post WooCommerce - Letters or Parcels?
You can force everything to be treated as a "parcel" if you like, even small things that would normally be classed as letters by australia post.
Please go to Plugins > Editor and choose the australia post plugin from the drop down menu at the top right.
Then on the right hand side choose the "australia_post.php" file
Then go to about line 291 where you see this line of code:
$calculate_products[$key]['is_letter'] = $is_letter;
change it to this:
$is_letter = false; $calculate_products[$key]['is_letter'] = $is_letter;
the system will try to fit an item into a "letter" when the product is less than or equal to 2 x 26 x 36 cm and weighs less than 500g
These are the limits set by australia post as seen here: http://auspost.com.au/parcels-mail/domestic-letters.html
If the product is larger than 2 x 26 x 36 cm, or heavier than 500g then it will try to fit the product into a parcel.
If you want the system to always return "parcel" prices, and not letters, then make sure your products are larger than 2 x 26 x 36 cm (example: 4 x 10 x 10 cm would be a parcel, 1 x 10 x 10 would be a letter).