Hello there, aadnanshah92,
You can add a Product/Service item and name it as freight charges. You'll be able to add it on the purchase order.
Here's how to create a new item:
I have attached a screenshot below for the sample PO. You can also check this article to know more about PO: https://community.intuit.com/articles/1772848.
Leave a comment below if you have additional questions.