Hi @shillongtitude,
If you have created the invoice in CRM, you can change the status from Unpaid to Paid. Once that’s updated, you can download the invoice as PDF and/or emailing it; the invoice will show the updated Paid status on a green box at the top of it.
In the screenshot linked here you can see a test invoice I marked as Paid in my test site.
If, on the other hand, the invoice is not being generated by CRM, there are different options I can consider suggesting. Do you have a WooCommerce store, for example?
As for the automation, we already have a feature request in place for our developers to consider, so I added your +1 on this ??