A package can sometimes be returned by the carrier, due to insufficient postage. This usually happens if your marketplace has incorrect weights associated with the products purchased by customers, which are then imported to your Appath account, or if an incorrect entry has been made in the weight section, on the shipping label.
I’m afraid that your Appath account cannot provide you with a way to print additional stand-alone postage that can be added to the package.
Therefore, in order to correct this, you will need to simply void the original shipment, enter the correct weight and re-ship the order, printing a new shipping label with the correct package weight.