You can request this feature formally with our development team by filling in the information within this link - http://prod.atg.quickbooks.intuit.com/commerce/common/fragments/feedback.jsp
Our online product will allow you to create, and save an invoice without e-mailing it; but this is tailored more towards Small-to-medium businesses as opposed to self employed users, the feedback is your best bet for including it in the product you're using.
In these occasions, the advice i give to my customers is to put your own email address in the email box; that way you can just bulk-delete any you've sent to yourself, or, even better; you've got a further backed up copy of your invoices within your email account which you can archive - which is great if you ever have HMRC wanting copies of information.
Thanks,
