create invoices

I need to create invoices - basically the customer registers and then someone else wants me to send them an invoice, I get paid, I need to go back into opencart and mark it paid, is there a plugin or method for creating invoices for customers?

Thanks
Rebecca

Same here. Standing by.

well, OC is a Shop Software, and an Admin can always create a Sale,
so, an invoice would be created by default. But an Invoice cannot be
created out of nothing, it would make no sense...
Ernie

But logically, that's the "reason" invoices are created -> for products. Or am I missing something here?