WordPress shortcode display as plain text

First of all as far as I know that you can’t call get_template_part() from your plugin.It’s a Theme only function . Try to keep your calculator code’s in a php file inside your plugin’s directory and include it like bellow:

function info_box_calculator_core(){
    include( plugin_dir_path( __FILE__ ) . 'mydir/calculator.php');
    //replace 'mydir/calculator.php' with your file name 
}
add_shortcode('info_box_calculator', 'info_box_calculator_core');

Let me know if it works for you.