The third available option when Discounting is to create a Discount code that can be used internally when generating a Quote.
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Simply go to Settings > Discounts > + New Discount to create your Discount Code:

As shown in the example below, simply enter the Discount Code name, the chosen code to use, select the appropriate Discount type (Percentage or Amount) from the dropdown menu, and whether the Discount will be applied to only the One-Time Fees, Recurring Fees or both the One-Time and Recurring Fees:

Simply select the appropriate code from the dropdown menu when creating or editing a Quote under the Discount section found in the Line Items Section. When using a Discount Code, the Discount Code Name will be displayed under the Item column with the discount dollar amount calculated and shown under both the price and total columns as per the example below:

If you'd like to have the Percentage displayed, simply add the percentage to the Discount Code Name to have it displayed on the Quote.