include and obvious keyword in the billing e-mail. It is too easy for the invoice to get lost in a flood of spam if it is not attended to immediately.
I searched for invoice, bill, the vendors name - searched for E&E, but the vendors name in the e-mail was "E & E", and quicken, instead of quickbooks. Something perhaps with a hashtag or other special character, so that I can quickly learn it in a distracted world.