As you enter shipped quantities via the Fulfillment module, “Shipment” records are created automatically (which you can view via ApeksOPS > Fulfillment > View Shipments). To create an AR invoice entry for a shipment, merely edit the shipment and set Invoiced to Yes.

Alternatively, to enter an invoice manually, go to ApeksOPS > Customer and click the “Account” link for a customer. Then click “Add Entry”.