When an order has to go out in more than one box, SKULabs can lay out the packages for you before the packer starts: with your own rule (Split items into packages), or with a recommendation (Suggest Packing).
Split items into packages (shipping rule action)
Go to Settings > Shipping > Rules and add or edit a rule, with filters for the orders it should apply to.
Add the action Split items into packages.
Add one or more groups. Each group matches items by item tag, item SKU, kit tag or kit SKU. Optionally pick the box for the group and the maximum number of units per box.
Save.
When the ship window opens, SKULabs splits the unshipped items into packages:
Groups are checked in order, and the first group that matches an item takes it.
If a group has a maximum per box, extra units go into more boxes of the same kind.
Items that no group matches go together in their own package.
Kit components stay together in the same package. A kit tag or kit SKU group takes the whole kit, and an item group only takes items sold on their own, not items inside a kit.
Example: a group for the item tag "Glass" with a maximum of 2 per box, and a second group for everything tagged "Apparel" in a poly mailer. An order with 5 glasses and 3 shirts opens with 3 boxes of glasses and 1 mailer of shirts.
Suggest Packing (add-on)
With the packing suggestions add-on, the ship window has a Suggest Packing button next to Rates. It recommends how many packages to use, which of your boxes or carrier package types each one is, and which items go in each. The panel shows how full each box is and how confident the suggestion is. Click Apply to shipment to fill in the packages, or close the panel to keep your own.
Suggestions are only as good as your item weights and dimensions and your box sizes, so keep those up to date. To add packing suggestions to your account, contact SKULabs support.
See also How to create and use shipping rules and Multi-piece shipping.