I concur, with all the above comments, I cc myself with a copy of invoices I send out from QBO,  and when trying to find one with only just INV no in the subject is at best hit or miss or very laborious.   You have fields [First] [Last] in greeting for the the body of the email,  surely it can't be that difficult to make available in the Subject line?!  Besides it more friendly to the customer to have there name or company in the subject line!