PO Invoice is matching my Descriptor amount into the Shipping section

The most likely cause for this problem is that the Descriptor is of Type 'SHIP' and should not be.  

This can be verified/changed under (Inventory Descriptors > Detail tab > TYPE field).  The 'SHIP' Type should really only be used for Descriptors that are used to pre-specify shipping charges as line items in the PO.  A lot of companies never pre-specify shipping charges in PO's, so it's not actually a common Descriptor Type.

Instead, you may consider setting the Descriptor to Type 'MISC' which should be used for things like supplies (including shipping supplies).