The ATO accept 2 methods of calculating GST for an invoice.
The first method is to calculate the GST based on the order total. This is suitable only in scenarios where 100% of the invoice incurs GST.
The second method is to calculate the GST on each item individually. This method is required where some items within the invoice may not incur GST, such as basic food items. This is the method used by FoodStorm since this is a likely scenario.
The reason the GST is not 10% of the total, even if all items in the invoice incur GST, is due to rounding. The ATO stipulate that the GST on a supply must be rounded to the nearest whole cent:
Furthermore, FoodStorm uses this method of calculation for compatibility with major accounting software packages, such as MYOB & QuickBooks, as they use the same method of calculation.
QuickBooks have a support article which details this further:
More information on valid tax invoices is also available on the ATO website: