You can use a Custom Invoice ID to your invoices. To assign a custom ID to your agency account, go to Settings > Invoices, Offer and Payments > Invoices. You'll have the option to start with a new number or continue an existing one.
Once you create an invoice draft, the invoice number will show up as code (###) until you issue or bill the invoice. This will protect the invoice number to be deleted by mistake.
Clicking "Bill" will create the custom number that can't be deleted anymore and will consider the invoice as "Created," it will also give you a link to the invoice in case you need to send it to the payer externally (i.e. WhatsApp, Messenger, Email, etc).
Invoice Event Title and Invoice Expense Groups
You can configure the invoice event title (description) as well as the expense groups with the expense type and a description (i.e. Flights - Antonio's flight from LAX to SFO)