to have taxable and non taxable items in the cart, you will need to add a new column to the eCart object named taxable
in the add to cart button, set the value for the taxable column to 1 for items that should be taxed and 0 for items that should not be taxed
Then in the eCart object, set the tax rule up on the tax tab.
Set the trigger to "Number of items With A Specific Column value":
If the total Number of items where 'taxable' has a value of '1' is '>' than a value of '0'
Set the calculation to "based on items with a specific value":
Total of 'TotalPrice' where 'taxable' has a value of '1' 'times' '.10'
replace '.10' with the correct tax rate you wish to use.