An Invoice can only be deleted if it has not been sent to the Customer. Once it’s sent, the invoice status will change to ‘Pending Payment’ and the Invoice can no longer be deleted. 
For invoices which have this  ‘Pending Payment’ status, or those Invoices already marked as ‘Paid’,  the delete function will add them into an inactive Invoices list, separate from your active invoices. Yardbook will notify you when you try to delete an Invoice with ‘Pending Payment’ or ‘Paid’ status. 

To delete an Invoice that has not yet been sent to the Customer, simply navigate to the Invoice, and from the ‘Actions’ drop down menu, select ‘Delete’.