I know this is a bit of a rant, but better to get it off my chest here where nobody will read it (except maybe my mum?), rather than rage-mailing every supplier who invoices me.
The basic problem is that everyone you send an invoice to has a system with which they attend to these accounts payable. So your objective is to be compatible with whatever process someone is using. Of course you can't be perfect for everyone, but there's a few things you can do to make it easy for most of us.
send invoices from a unique address
Don't send me an invoice from `firstname.lastname@example.org
email@example.com`, because I can't simply forward all emails from that address to my bookkeeper in case you actually send me something important. Yeah I can make a filter to match on subjects as well as senders, but those are more brittle and break every time you change your subject line.
send invoices as attachments
Anyone who's ever tried to send any kind of automated email knows that email formatting sucks. It's like trying to draw a flow chart on a teletype machine. Worse, every single recipient you send that to is going to need to convert it to a pdf. Whatever service you provide, I'm sure you don't provide it on a "some assembly required" basis, so your invoices should be no different.