add a new column to the eCart object for taxable.
in the add to cart behaviors, set the taxable column to 1 for items that should be included in the tax rule, and 0 for items that should not.
in the tax rule, use "Number of items with a specific column value": Number Ot Items where "taxable" has a value of "1" is ">" a value of "0" for the trigger
in the calculation, use "BAsed on Items with a specific column value": Total of "Total Price" where "Taxable" has a value of "1" "Times" "0.20"