Hey there @Jay888,
Thanks for joining us here in the community! We are so happy you're with us.
To my knowledge this is not an option for label messages currently. I'll move this over to our idea section so that other community members can show their support with kudos!
I was looking through all the available options for those lines, and I see there's a "SKU #1" and "Fill SKU #1" for example. What's the difference between those?
That "Fill SKU" that you're seeing is the "Fulfillment SKU" blank from the product record for that individual item. Often but not always a spot for FBA reference numbers.