Hello Rachel,

Let me guide you on how to add GST to your invoices in QuickBooks Online. First, let's enable your GST to apply codes to your invoices. Here's how:

1. Click on the GST tab.
2. If you haven't set up GST, click on Get started button.
3. Set your GST filing frequency and accounting method.
4. Select other GST obligations you need to report based from ATO mandated for your business.
5. Click on Save and finish.

Once done, let's open an invoice you've created on file and assign GST code on them. Here's how:

1. Click on the Sales tab.
2. Click on Invoices.
3. Locate and select the transaction and click on Edit. 
4. Under Amounts are field, set a GST code like Exclusive or Inclusive of tax.
5. Under GST column select GST 10% tax code.
6. Check the summary of the invoice.
7. Click on Save or Save and close.

That's it! You've now assigned GST code to your invoice. Repeat the same process for other invoices you need to assign GST code on them.

Check out this link to learn more on how to: Set up GST and BAS in QuickBooks Online.

Drop by again in the Community if you have other questions about invoices. I am around to help you.