Define a QWeb report template for your custom label.

py.file

class StockPicking(models.Model):

    _inherit = 'stock.picking'

    @api.multi

    def action_done(self):

        super(StockPicking, self).action_done()

        # Check if the package is done

        for package in self.package_ids:

            if package.is_done:

                # Print custom label

                self.env.ref('your_module.custom_package_label_report').report_action(self)