Allows to enable the invoice support in LemonStand.
By default invoices are disabled, and Administration Area user interface does not display any controls and labels
related to invoices. Some modules, for example
Subscriptions Module,
require invoice support and this event allows them
to enable it.
public function subscribeEvents()
{
Backend::$events->addEvent('shop:onInvoiceSystemSupported', $this, 'process_invoice_system_supported');
}
public function process_invoice_system_supported()
{
return true;
}