Can I make a discount per line and not for the total transaction?

Yes, adding discount per line item is a breeze, @usersales5.

You can create a new service or non-inventory item for the discount. Then, ensure that the amount is negative. To get started, please refer to these steps:

1. Go to the Gear icon and click Product and services.
2. Hit New. 
3. Choose either Non-inventory or Service. In the screenshot, I chose the Service.
4. Enter the name for the discount item.
5. Assign a category. This should be the item you are selling and giving a discount.
6. From the Sales price/rate field, input the discount as a negative amount.
7. In the Income account drop-down menu, select the Discount given account.
8. Click Nontaxable, in the Sales tax category drop-down.
9. Once done, tap Save and close. 

Then, here's how to create a transaction with the discount item:

1. Select the +New button and choose Invoice.
2. Enter the customer's name.
3. Choose the products and services. Then, add the Discount item you just created. You can change the amount if you need to, just make sure it's negative.
4. Fill in the other necessary field.
5. Once finished, hit Save and send or Save and new. 

For your reference, please visit this page: Add a discount to an invoice or sales receipt in QuickBooks Online. 

Also, QuickBooks makes it easy to process payments from your customers. For the detailed steps, please see this article: Record invoice payments in QuickBooks Online. 

If you have further concerns creating a discount for your items in QuickBooks, please leave a reply below. I'm just a post away. Have a good one.