If you want an idea of what the process is then look here under “Purchase Orders” https://blog.asmartbear.com/selling-to-large-companies.html
As above - would definitely NOT do monthly, annual only and then only above certain $ value (for me $500 but I occasionally make exceptions).
You may be tempted by certain payment processors that say they can handle PO’s - but don’t be fooled they will do an automated attempt at collecting payment then give up easily leaving the problem with you again.
You may think that you can just turn off whatever service the company has to force payment but whilst this is a good last resort doing it too often will piss off your real customer as they will be equally baffled and impotent in the face of the byzantine bureaucracy and deliberate incompetence that goes on inside large company accounts payable departments.