The form created by uc_shipping_shipment_edit() has editable fields for changing the origin and destination e-mail addresses. Also, for each package, it has fields for entering a custom weight.
However, when you click the "Save shipment" button, uc_shipping_shipment_edit_submit() does not save the values in these fields. Anything you type is lost. This is not due to a bad line of code. It just doesn't have any code to save the shipment information.
This is probably related to the fact that the uc_shipments table doesn't have any fields for storing e-mail addresses and the uc_packages table doesn't have any fields for saving package weights.
Doing this properly would require updating the schema for packages and shipments.
The fields are still useful, as uc_shipping_shipment_edit() can be used as the basis of a form for a custom module that might store the values in parallel tables or use the values directly to generate a label or such.