Da gubmint requires that, sometimes, we must play by their rules. And sometimes, some people choose to make those rules more complicated than they need to be. So this is a bit of a "how to" to make the simplest possible invoice.
It has to have the word "INVOICE" at the top, and it has to be all caps.
It needs to have an invoice date. It literally says "invoice date:" followed by the date that the invoice was created.
It needs to have a unique invoice number. It literally says "invoice number:" followed by a number. I usually use the date in a format like this "20150606". If I ever send two invoices on the same day, the second one will be "20150606B".
It needs a spot that says "terms:" followed by all sorts of things. I usually put "terms: net 7" - this is relatively standard for an invoice. It means "don't take any discounts and pay within 7 days." You might be tempted to fuck with this, but don't fuck with this unless you REALLY know what you are doing. You might even think about leaving this out - it MUST be there. It is a critical part of an invoice.
Near the top is "Bill to:" followed by whoever is gonna part with coin.
Near the bottom of the document is "TOTAL DUE:" and a dollar figure. Usually to the right.
At the bottom to the left is "remit to:" which is info about who is getting the coin and how they are getting it.
So here is a sample invoice. Yes, there are hundreds of things that can be done differently for all sorts of reasons, but you probably don't want to get into that.
Bill to: Justin Case
INVOICE DATE: March 18, 2015
INVOICE NUMBER: 20150318
terms: net 7 days
You can just copy and paste this into the body of an email and edit it as needed. I keep a file handy with my particulars and copy and paste it onto an email and then edit the email. I've done this a few hundred times.
I cannot think of how to make this invoice any simpler. This is the bare bones. And it gets the job done spectacularly.