How can I use discounts to write off small amounts on my customer invoice on quickbooks online (latest version)?

I'll share with you the two methods on how to write off small amounts on the invoice in QuickBooks Online (QBO), @kan-yama-kan. 

Currently, using discounts to write off the invoice's small amounts is an available option if you're connected with an accountant user. I'd suggest having them utilize the Write Off Invoices tool. This way, they can apply discounts to zero out your invoices. QBO will automatically post them to the appropriate write off account. Also, it updates your accounts receivable and expense balances. For more details, have them visit this article: Use Write Off Invoices. This link is specific to the US region, but it also applies to your country. 

If your QBO account isn't connected with an accountant user, we're unable to use discounts for the write off process. However, we can first set up a bad debts expense account from the Chart of Accounts menu. Then, create a non-inventory bad debts item from the Products and Services page. Just select Bad debts as the Income account for this. 

Once done, record a credit note for the bad debt so you can apply it to the invoice. Doing so will help you write off the invoice's small amounts. Also, your accounts receivable and net income will stay up to date. I'll guide you how. 

To enter a credit note: 

1. Go to + New from the left menu. 
2. Select Credit note or Give credit under Customers. 
3. Choose the appropriate customer. 
4. Select the Bad debts item you've created in the Product/Service column. 
5. Enter the write off amount in the Amount column. 
6. Type in Bad Debt in the Message displayed on statement box.
7. Click Save and close.

The screenshot below shows you the last five steps. 

To apply the credit note to the invoice:

1. Go to + New from the left menu. 
2. Select Receive payment under Customers. 
3. Choose the appropriate customer. 
4. Check the box beside the invoice from the Outstanding Transactions section. 
5. From the Credits section, check the box beside the credit note.
6. Click Save and close.

I've attached a screenshot below to show you the last three steps. For the detailed instructions, see Steps 2 - 5 through this article: Writing Off Bad Debt in QBO. 

I recommend running the Account QuickReport of the bad debts expense account. It helps you track all the receivables you tagged as bad debt. Just go to the Chart of Accounts menu and enter its name in the Search box. Then, click Run report from the Action column. 

I'm just a post away if you need anything else. Keep safe always, @kan-yama-kan.