An invoice can only be deleted if it is at draft status. To delete an invoice by:
In the contact,> invoices tab using one of the following methods: Single row - Click the invoice row action button against a draft invoice status row and select 'delete'. Multi-row - Select the check box(es) on the row left for each item you want to delete. Then click the multiple rows action button and choose 'delete'.
The list refreshes and the item(s) have been deleted.
Tip: If the invoice has already been finalized, then it cannot be deleted. Try using the cancel or credit the invoice options.