How can I, while making an invoice, add item-wise discounts by percentage for specific products?

Thanks for joining QuickBooks Community family, userezrajlyoung.

The Item-wise discount isn't available to all versions. Instead, you can turn on the discount feature and apply a specific discount amount or percentage to the entire sales transaction.

1. Go to the Gear icon and select Account and Settings.
2. From the Sales tab, select the Sales form content section.
3. Select the Discount checkbox to turn it on.
4. Tap Save, then Done.

Once you create an invoice or sales receipt, select either Discount percentage or Discount value to enter the discount as a percentage or a specific amount.

Another way is to create a service item. You'll have to manually calculate it since we can't enter a percentage into this method.

1. Go to the Sales tab, select Products and Services, then click New.
2. Select Non-inventory or Service for the type.
3. Name the item Discount. In the Sales price/rate field, enter the discount as a negative amount. 
4. From the Income account drop-down menu, select the Discount given account. QuickBooks creates this account for you when you turn on the discount feature.
5. Tap Save and close.

Then, add it as a separate line item to the sales transactions.

Leave a comment below if you need anything else. Wishing you all the best!