I don't know if this is fully correct. through experimenting, it appears you must click Payment on the POS side as you have said. What I have noticed is that this does not actually do anything on the accounting side. Once you click invoice on the POS order, the customer invoice is created in the exact same process for a regular sales order. So it appears you are safe to add payment under POS and then register the correct payment at some later date with the accounting invoice.

I know that this is fully correct because I've developed several modules for POS. The option that you're talking about doesn't change this. If you create an order without invoicing it you're going to see on the backend that the order has a button to invoice that order but it's already paid. This is only to create the assets. Payment is already done.

I don't fully understand what you mean. I'm saying the payment doesn't ever appear anywhere in accounting. How can the payment be done if it is not in accounting and reflected in any account? Regardless, I'm saying if you create an invoice you can process the payment the regular way. Is this different?

Yes, this is really different. The payment is going to be processed only when you close the POS session.

But it's already there, you can check for that on POS Session.

Ok. I guess it's back to the drawing board. I didn't actually close the session.