This happens when you have auto-fulfil on and use packing slips.
The reason is happens is because the shipping protection is being fulfilled separately from the rest of the order, so Shopify creates an additional fulfilment for it and that’s what leads to the extra packing slip.
Unfortunately, Unicorn doesn’t have any control over how Shopify formats or generates these packing slips, as that’s handled entirely within Shopify’s fulfilment system.
Unicorn just tells Shopify to fulfil the item.
That said, if you'd prefer everything to be on one packing slip, one option is to disable auto fulfilment for the shipping protection product.
That way, it can be fulfilled together with the rest of the order, and Shopify should generate just one slip instead of two. You can also speak to Shopify support and see if there is a way to turn off the additional packing slip.