To mark a payment as NSF you would go to the Billing tab > Payments. From here, you will click the white drop down across from the payment and select Mark as NSF. There will be a pop-up asking if you wish to create an invoice for a NSF fee. If you do not have a NSF fee click Cancel, otherwise if you click OK, it will allow you to create an invoice on the spot. If the amount populated is not correct, edit the amount and click Save. If it is correct, then you will just click Save. Now in two easy steps you have marked a payment as NSF and created an invoice for the NSF fee!