You can cancel your unused shipping label and receive a refund by tapping on "Shipping Label Options" at the bottom of the Order Detail screen. Once you tap on the button, you should see a "Refund my Label" option. After it has been refunded, you can purchase a new label if needed.
If you want to split the order into 2 shipments, or the order isn't fitting into one package, you can contact support with the size labels you need and we can get those sent to you. If you do end up shipping the order in multiple package, be sure to the let the buyer know so they are not questioning why they didn't receive all of their items in case one package takes longer.