Hello there!
You project would be a perfect fit for Google Apps for Business. This is a paid service, but it allows sending 2000 emails a day, as opposite to the free version which only allows 100. Right now it costs 5euro a month. As far as I know, most of free email services have daily limits, so if you haven't hit it yet, you probably will soon.
So the system will look as follows.
You fill in all the data for customers into google spreadsheet, then press a custom menu item, say we call it "send invoices", the custom script will pick up all the data, fill it into the predefined doc template (stored right there in Google Drive), saves all the files as PDF (potentially into the individually named customer folders), then creates the email with predefined text, attaches the PDF and sends it out.
Once that is setup, you can develop you system further with automation and integration with things like calendar, forms, graphs, analytics etc.
There is a reason I recommend this solution. To send e-mails you need to be online in any case, so instead of having native application running on your Mac, you can do it all online. No need for additional backup, no need to be on specific computer or operating system. You have a browser = you can do your invoices.
Let me know if this option interests you.
Cheers,
Denis