Manually Entered Freight Amounts in SO/SO Shipping Data Entry?

We have a 3rd party shipping program integrated with Sage, but for some SO's, we need to be able to simply specify a freight amount and had it flow through to the invoice.  I'm trying the different types of freight calculations, but no matter what, once clicking on the Shipping tab in Shipping Data Entry, the freight is zeroed out.  Is there a way to configure a shipping rate that will simply charge on the invoice what was entered on the SO?  Thanks in advance for any help?