With the automation "Update order line, it's possible to set a default weight, split evenly amongst the order lines.
You will need to select "weight" as the attribute to update.
In the value field, you need to insert the following Webshipper Template Language (WTL) snippet:
Replace the number "5000" with the needed default weight in grams.
In the above case, the total weight will be 5kgs or 5000 grams.
This is how the automation should look: