You can easily change the order that your labels print in ShipStation by setting the Label Document Options.
Head to your Account Settings (the wrench icon in the upper right).
Next, select Printing and then Printing Setup from the sidebar on the left.
Select the link underneath the Document Options header for Labels. This window will show you all customizations that can be made to how your labels print.
Scroll to the Order Labels By section and click on the drop-down menu to view your options.
Once you’ve selected your preference, be sure to Save Changes at the bottom of the page. Now your labels will print in your selected order. The Packing Slip order is also controlled by this setting, so that you can easily match them to each other.