In the upper right corner, select the Actions dropdown → then select "Create Label".
A pop up modal will appear.
In this modal, you can edit the Ship Date on the label you are purchasing if you wish, and the name of the batch you are creating. Both have default values applied to select the current day you are buying the label on / with.
Select Next and Submit.
Your labels are now being purchased. Once complete, you can download the label(s) and collated files from either the Order directly, or from the Batch of Orders you purchased together.
Tips
You can select multiple orders at once to batch-create labels together.
Batch name defaults to the current date — rename it if you want something more identifiable (e.g., client + AM shift).