In v6, you can add freight at the customer record and/or sales order record. Entry onto those documents will flow through to the respective invoices. On customers, you enter it on the Commercial tab and on the sales order it is on the Invoicing tab. Entering it on the customer record will give you a default amount on each sales order that you can adjust.

More on this, these are called invoice elements in X3, they are extra charges that can be added to a sales order/sales invoice and are quite useful.

The first step is creating the invoice element: Setup / Sales / Invoice Elements and an accounting code for the financial activity.  If you setup a default value here it will carry through to the order/invoice.  Once you are done press the control button at the bottom.

Finally you have to select which documents it will appear on using the Element Position button, heres a snip from my test environment.

Now in sales orders/invoices you will see the following item:

If you entered a default value then it will appear.  Additionally you can use sales price lists to set the default value based on customer category or whatever criteria you can put into a pricelist.

Just as an endnote: Be very careful if you are editing these (adding new ones is not a problem), I am not sure what happens if you change the order of invoice elements once the system has been running.  I always use a test folder to see what happens when I want to make changes to invoice elements.  Our implementer said if we wanted to add a new invoice element between 1 and 2 it could be problematic but he didn't expand on it any more.