How to add weight to a single product
From you Shopify store dashboard, click Products.
From the list of products click the item you would like to edit.
Scroll down to 'Weight' in the 'Shipping' section.
Make sure to check 'This is a physical product'.
Then enter the weight.
Scroll down and click Save.
Take note of what unit you are entering weights in. Are you entering your weight in kilograms or grams? If entering the weight in grams, make sure not to enter a value with a decimal point, Australia Post does not accommodate amounts of less than 1 gram.