Hello, thanks for your reply.
Yes, I'm trying to override the actual logic file.
The changes are in the functions calculate_item_totals() and calculate_discounts() in 'class-wc-cart-totals.php' but I couldn't find a way(filter/actions) to change what I want.
Change in calculate_item_totals(): replace $item->total + $item->price with $item->price
$total_taxes = apply_filters( 'woocommerce_calculate_item_totals_taxes', WC_Tax::calc_tax( $item->total + $item->price, $item->tax_rates, $item->price_includes_tax ), $item, $this );
Change in calculate_discounts(): Comment-out the lines where it applies taxes to coupon.
I tried to use woocommerce_calculate_totals hook but couldn't manage to change the tax totals as I want it. Do you have any tips on how to accomplish that?