The ISN allows for you to add payment to an order as a Post Sale Payment. The Post Sale Payment function is for a credit card payment processed outside of ISN, however the order needs to be shown as paid.
Go to Inspections -- View Inspections -- Actions -- Payment Information or by clicking the $ icon if you have those enabled.
Once on the payment page, click on the Post Sale Payment button/icon:
The following will appear:
This will allow you to add that payment was received for the order, outside of ISN!