GroovePacker has a "Special Instructions for Packer" option that allows a message to be displayed to the packer when an item is being packed. This message is displayed every time the item appears in any order.
You can add packing instructions to a product directly in GroovePacker (open a product page by clicking it's name in the products list, then click the Scan & Pack Options tab to find the message text area) or if you're going to be adding instructions to many products at once you can use the "special instructions" field in the Product CSV importer.
If desired you can also request that the packer confirm that they have seen the instruction by scanning or entering their confirmation code. To do so just enable the "Packing Instructions Confirmation" switch. This is only recommended for critical instructions that warrant slowing down the packing process to confirm.