PLUGIN FIX please read

Thanks to JasperPress

function filter_session($session_data, $value, $key){
      	      if ($value['sample']){
      	      	      $session_data['sample'] = true;
      	      	      $session_data['unique_key'] = $value['unique_key'];
      	      	      //$session_data['data']->price = 0;
					  $product_id = $session_data['product_id'];
					  $sample_price_mode = get_post_meta($product_id, 'sample_price_mode', true) ? get_post_meta($product_id, 'sample_price_mode', true) : 'default';
					  $sample_price = get_post_meta($product_id, 'sample_price', true) ? get_post_meta($product_id, 'sample_price', true) : 0;
					  if ($sample_price_mode === 'custom'){
					  	$session_data['data']->set_price( $sample_price );
					  }else if ($sample_price_mode === 'free'){
					  	$session_data['data']->set_price( '0' );
					  }else{
					  	//default
					  }
      	      }
      	      return $session_data;
      }

I modified line 210 and 212 from:

210: $session_data['data']->price = $sample_price;
212: $session_data['data']->price = 0;

to:

210: $session_data['data']->set_price( $sample_price );
212: $session_data['data']->set_price( '0' );