Can I add amount instead of Tax code in invoice and show total before tax and total after tax to comply with SG Regulation , show total before gst and after gst tax.

Glad to see you here, @listari.

The option to directly add an amount instead of a tax code on an invoice isn't available. What we can do is add a new service item for the tax amount, then select a GST rate for your invoice.

Follow these steps to create a service item for the tax amount you'd like to apply to your invoice. 

1. Go to the Gear icon, and select Products & services.
2. Select New, then choose Service.
3. Add a name, then tick the I sell this product/service to my customers checkbox.
4. Enter an amount in the Sales price/rate field, or you can leave the Sales price/rate data field blank.
5. Select the Income account ▼ dropdown and the account you want to use to track the sale.
6. If you need to track sales tax, select tax from the Tax▼ dropdown.
7. When you're done, select Save and close.

Once done, you can generate your invoices and add this as an additional item. Then, select the GST of the item and edit the GST amount if necessary.

Regarding the invoice template, you're right that you cannot edit the Total text in an invoice. You can visit this article to learn more about what you can customise in your sales forms: Customise invoices, estimates, and sales receipts in QuickBooks Online.

In addition, please refer to this article for more information about filing your GST return in QBO: File your GST return and record tax payment in QuickBooks Online.

You can always leave a comment below if you have further questions about your tax rates. I'll be around to help you out. Keep safe!