Here are simple steps to Add "Tax on Tax" in Moon Invoice:

1. Open the Moon Invoice app.
2. Tap on Companies from the main menu.
3. Select your company and tap on Taxes.
   
4. Tap on the ➕ (plus) icon.
5. Choose New Tax.
   
   
6. Fill in:
   
   • Tax Name (e.g., GST or PST).
   • Tax Percentage.
   • Enable the "Tax on Tax" option (this applies the new tax on top of an existing tax).
     
7. Select the base tax you want to apply (e.g., applying PST on GST).
8. Lastly, tap on the ‘Save’ button.
                                                                                                                                          Moon Invoice