If you have a one tick upsell in your cart such as shipping protection or a digital product (e.g. an ebook), then you can auto-fulfil the item within the cart settings. This means you won't need to manually fulfil the item and it won't flow through to your 3PL as it's a non-shippable item.
To auto-fulfil a One Tick Upsell (e.g. shipping protection), go to your One Tick Upsell Cart block and in the settings, ensure this checkbox is ticked:
'auto fulfill the item - use for virtual products only e.g. shipping protection'
When this is ticked, the item will be fulfilled instantly.
Please note: if you use packing slips it may still appear on the packing slip because it's a product. This is a Shopify limitation and Unicorn is not able to influence the packing slip contents.