Thanks for coming back, angeline-soh-ncs.

When creating the invoice, make sure you have selected the appropriate GST tax code that corresponds to the 8% GST rate. You can assign tax codes to individual line items on the invoice in QuickBooks Online (QBO). Then, we can review each line item and make sure the correct GST tax code is assigned.

Just in case you have followed these steps and the 8% GST is still not applying correctly, I'd suggest checking the following:

1. Verify that the customer associated with the invoice is set up with the correct tax settings. Ensure that the customer is set to the appropriate GST category that applies the 8% rate.
2. Confirm that the transaction date on the invoice is within the effective dates of the 8% GST rate. If the date falls outside the effective period, the system may not apply the correct rate.

To modify GST rates in QuickBooks Online, you can use this article as a reference: Edit GST rate in QuickBooks Online. For choosing either Total value of taxable purchases excluding GST"? or should it be "N/A", I'd suggest consulting your accountant to properly guide you with this one. Also, please take note of some reminders below: 

• Existing transactions that used the earlier rate remain unchanged unless you edit the transaction and specifically select a different GST rate.
• For recurring templates that use the changed component, new transactions created with the template use the new rate. Existing transactions created with the template and the earlier rate remain unchanged.
• The new rate will be available for new transactions entered after the Effective date. 
• If you create a new transaction dated before the set Effective date, the transaction uses the old rate. 
• If you create a new transaction but use dates in the past, the transaction uses the new rate. 

You can also check out our newly updated GST rates in QuickBooks Online Singapore in this link: 9% GST rates in QuickBooks Online Singapore.

Feel free to come back to this post if you have other concerns regarding editing the GST tax rate and managing your VAT in QBO. I'll be here to lend a helping hand. You have a good one.