Here is how you can add tax in counter:
Step 1: Click on counter at the bottom of the screen
Step 2: Click on 'New Sale (+)'
Step 3: Choose the category from which you want to add an item to the counter
Step 4: Click on item which you want to add to the counter
Step 5: Choose the number of units you want add to the counter ( (+) for addition of an item and (-) to remove)
Step 6: Click on 'Go to the Counter'
Step 7: Click on Add Tax (under grand total)
Step 8: Add the tax charge and click on save
Note: If you already include tax on an item, by default Rs.0 will be displayed when you try to add tax
Related articles: