When you add a Product to your Opportunity, you have the option of entering a Line Description:

If you enter a description here that is the description that will appear on your Saasu Invoice.

If you do NOT enter a description here and the Salesforce Product has a Description, that description will be copied to this Line Description automatically and your Saasu Invoice will show this description.

Here is a screenshot of an Opportunity:

1. Product Chair we did not enter a Line Description, and a Product Description exists, so it is copied here
2. Product Table we did not enter a Line Description + there is no Product Description for this Product
3. Product Table we entered a Line Description when we added it to the Opportunity

Our Saasu Invoice looks like this: