To apply a commercial discount, simply create a new line on the quote and label it as a commercial discount. You can add it to an existing section if the discount applies to a specific room or category, or you can create a new section named "Commercial Discount," for example, which will allow you to add multiple items if you need to detail certain discounts.
Then, to apply the discount, enter a negative price on the line(s) for the commercial discount, and the total of the quote will update automatically.