To delete invoices you can either go to the Billing tab > Invoices > Select the check box(es) beside the invoice(s) > Click on the top drop down menu > Select Delete Invoices. 

Or, you can go to Overview > Go to the payer's account >  click the down arrow in the payer's billing account > Click on the garbage can across from the invoice.

NOTE: You cannot delete an invoice if it still has a payment applied to it.