PayPal Invoice Generator Tool or other Payment provider?

Hey community, following question.
Am startet a week ago an Script-Installation Service. Now we have 20 Sales a day. My problem is the invoicing. Is a software or service on the market how can sends invoices to my clients. We only proceed by paypal.
Or is there a good system how has not highly coasts because we make under 10$ for 1 installation.
At the moment we make that all manuell and 500 invoices a month is realy hughe and for me is automation very important

See original post by Oranboard