Hello PatrickV,

Thanks for posting in the Community!

Since the GST in QuickBooks Online is automatically calculated at 10%, you'll have to manually edit the GST amount on the invoice summary field to have a GST amount of $90.

The total balance due may change if you manually edit the GST amount of the invoice, make sure to set the correct amount and tax code set to either Exclusive or Inclusive of tax in the invoice.

Post again in the Community if you have more questions. I'll be around to help you!