Let me share additional information, @waleed3.

You can consider using a discount service item to apply a discount to one or more items on the invoice. To do that, you'll need to create a discount service item first before applying it on the invoice. Once created, you can manually calculate the discount amount based on the percentage. 

1. Go to the Gear icon.
2. Select Products and Services.
3. At the top right, select New.
4. Choose Service for Products/Services type, then complete the information needed.
   • In the Name field, enter Discount. If you plan to create multiple discount percentages, you can enter the percentage and Discount for the name (Example: 10% Discount).
   • In the Sales price/rate field, enter the discount amount as a negative. (You can manually calculate the discount amount based on the percentage promo)
   • From the Income account drop-down, select Discounts given account.
   • In the Sales tax category section, it depends on your state if sales tax is calculated after discounts are applied then mark the discount line taxable.
5. Click Save and close.

Once done, you can now use the Discount item on an estimate. Here's how:

1. Click the Plus (+) icon.
2. Select Estimate.
3. Choose the appropriate customer.
4. Add the product and services, then enter the total amount of the item.
5. Select the Discount item on the next line.
6. Click Save and close.

You can always leave your comments below if you have other questions. I'm always here to help.