Create a “part” to quickly apply an NSF fee item for your trouble. Point the account revenue to NSF and track it accurately.
If you want to have an easy repeatable NSF charge mechanism, create a “part” to handle this. Create a part with a simple P/N for easy recall such as NSF or NSF1. Set account class to NSF (create as needed) and create a Category as well. This way the dollars collected will be counted under it’s own unique GL code and not interfere with parts reporting so much.
Create an invoice with this NSF charge on it and post as a charge, Then they can come in and settle all this business (orig charges + NSF fee) with you at once in Batch Payment window.