We're unable to directly customize packing slips to add SKUs in QuickBooks Online, KeelAP.
However, we can use the Description column of your transactions and then add your SKU details from there. You can refer to the screenshot below for your visual reference:
Once added, it will show up on your packing slips. Here's how:
Go to My Apps, select Customers from the Customer Hub.
Choose a customer, then tick the box of the transaction.
Click Batch Actions and select Print packing slips.