The best bet here is to try to identify the method that epicor uses to do the same action. Turn on tracing, perform the action that changes the shipvia using the standard epicor tools. This records the methods that epicor is using. Then use Jose’s trace parser Trace Helper Utility for Epicor ERP 10 - ERP 10 - Epicor User Help Forum (epiusers.help) to see which methods are firing. Once you identify the method, you can also use the trace parser to see which field need to be populated. There could be a few steps epicor takes other than just updating the text value of the field.
Good Luck!