Hey again, Hélène!
Oh, this is a tricky one because free transactions are automatically processed by both Event Tickets Plus and the ecommerce plugin it’s being used with.
One idea might be to try using WooCommerce with the plugin because it has settings that allow you to set the status of a transaction before it gets completed. That would allow you to set the status of any transaction to Pending before it gets completed, notify you as the administrator, then mark the transaction Complete before sending the ticket out to customers.
WooCommerce also has an extension called WooCommerce Order Status Manager that allows you to customize your order statuses even further, should you need it.
Cheers!
Geoff