This article explains how you can create and send an invoice in Billit in three easy steps.
Please note that you can only remove the last unsent invoice after creating it by clicking the 'Delete' button. It is in fact mandatory to make your invoice numbers follow one another.
If you still want to remove a created invoice after creating several other invoices, you need to create a credit note for that invoice. Otherwise the created invoice can't be matched in the accounts.