Applying Tax on only certain items
==================================
Steve Kneizys
12/18/00

Taxing only certain items isn't too hard.  Say your database user5 field
has "exempt" if tax exempt, and nothing if you need to charge the tax in
that state.  Then you'd have to loop through each of the items in the cart
and read the database entry for each to calculate the tax. 

Another way to do it is add a new field to the cart, and as each item is
loaded into the cart add what the tax would be if you are charging tax,
then decide if that value is valid at the end and loop through.  (On cart
versions 3.3c and above their are user fields to do this.) 

The easiest would be the first.  It should work on old and new versions of
agora.cgi and commerce.cgi.  In the sub calculate_sales_tax there is this
logic: 

                # A match results in the sales tax
                # being calculated.

if ($sc_sales_tax_form_variable ne "") {
  foreach $value (@sc_sales_tax_form_values) {
    if (($value =~
      /^$form_data{$sc_sales_tax_form_variable}$/i) &&
    ($form_data{$sc_sales_tax_form_variable} ne ""))  {
        $sales_tax = $subtotal * $sc_sales_tax;
      }
    }

At the top of that sub, after the last local statment, add:
local ($inx,$product_id,$my_subtotal,$temp);
local (@my_cart_fields,@my_db_row);
if (!($sc_db_lib_was_loaded =~ /yes/i)) {
  &require_supporting_libraries (__FILE__, __LINE__,    "$sc_db_lib_path");
 }

Those statements setup variables and load the database library if
it is not already loaded.  Then change the line in the above logic

  $sales_tax = $subtotal * $sc_sales_tax;

(actually there are two lines like that, go ahead and change both if you 
need to) to be this series of lines like this:

  open (CART, "$sc_cart_path") || &file_open_error("$sc_cart_path", 
	"calculate tax",__FILE__, __LINE__); 

   while (<CART>) {
     chop;
     $temp = $_;
     @my_cart_fields = split (/\|/, $temp);
     $inx = $cart{"price_after_options"};
     $my_subtotal = $my_cart_fields[$inx];
     $inx = $cart{"product_id"};
     $product_id = $my_cart_fields[$inx];
     &check_db_with_product_id($product_id,*my_db_row);
     $testval = $my_db_row[$db{"user5"}];
     if (!($testval =~ /exempt/i)) {
       $sales_tax = $sales_tax + $my_subtotal
        * $sc_sales_tax;
      }
    }
  close(CART);

Those loop through the cart and assign tax only if user5 field does
not have the word "exempt" in it.  If you need other flags in other
parts of the code, such as shipping, discounts, etc., you may want
to use logic similar to the above.  Having user5 be something like
"exempt nodiscount" could flag two different things.

Enjoy!