You can set default values for item custom fields in invoice by updating the product details.
Step 1: Go to Products and Select the product for which you want to add default values for the item.
Step 2: Click on the edit option from the top right corner
Step 3: Scroll down and enter the default value on the custom fields and Click update product
Step 4: Once updated, it will be shown like this.
Now, when you add the product to a invoice, the custom field will be pre-filled with the default value.