You can use the ‘Acrobat Reader DC’ to crop the shipping label to print.
Demo video: https://www.youtube.com/watch?v=9afZOcQ8TZw
*You can use the Free "Labelife" software to crop the shipping label print
If you are shipping more than a few labels, you would benefit from using a commercial solution that create 4 x 6 labels in one-click.
Official guide from Adobe: Print portion of a PDF page using Acrobat or Reader
How can I crop multiple pages in the same document at once?
1)On Adobe Acrobat you are able to crop several / all pages together.
Please see: https://helpx.adobe.com/acrobat/using/crop-pdf-pages.html
2) Briss is a free open-source application available for cropping PDF pages at once. It may take a little while getting used to it
but once you are comfortable with it, it is pretty fast: https://sourceforge.net/projects/briss/
3) For Mac:
There is a third-party company that offers a software for cropping labels
Please download form: https://www.peninsula-group.com
There is an operation tutorial video: https://www.youtube.com/watch?v=Mos4GNnRZ5c