Send form data to functions.php

The answer is AJAX. Each time the totals are calculated, send the form data to the wp ajax handler.

Step 1: pass your ajax URL from PHP to javascript using wp_localize_script just after you enqueue your script file.

wp_enqueue_script('myScript', 'my/script/path.js');
wp_localize_script('myScript', 'php_params', ['ajaxurl'=>admin_url('admin-ajax.php')]);

Step 2: make the AJAX call in your script.

$.ajax({
    type: "POST",
    url: php_params.ajaxurl,
    data: {
        action:'mytag_function_name',
        your:'data', 
        goes:'here'
    },
    success: function(data){
        // do something
    },
    error: function(errorThrown){
        // do something
    }
});

Step 3: add the ajax method to your functions.php file

add_action( 'wp_ajax_nopriv_mytag_function_name', 'mytag_function_name' );
add_action( 'wp_ajax_mytag_function_name', 'mytag_function_name' );

function mytag_function_name(){
    // do something with your $_POST data
    $post = filter_input_array(INPUT_POST);
    echo "Here's my result to get returned to Javascript";
    die();
}

Learn more about AJAX in the WordPress Codex:
https://codex.wordpress.org/AJAX

Hope this helps 🙂