Hi Mike,
I believe you might be missing a "Retrieve value from table" action. In the "Update field in Table" action, there are 5 ways to find the new value: Type In, Action Result, Value from Row, Value from Screen and Value from Session. Type In, Value from Screen and Value from Session we know won't work. That leaves Action Result and Value from Row. My hunch is that you're using Value from Row, which also won't work. The reason is that the row is the grid row that is filtered to only show Customers with ParentFullNames. These are your subaccounts and they don't hold the postal code you wish to use.
If you use the "Retrieve value from Table" first, you will be able to locate the proper record in the table then use that action result to populate the grid row. See below for an example.
Hope this helps,
- Adam