Currently SAP doesn't have the option of maintaining the order of the menu items in the correspondence button. The SAP code that prepares this drop down menu performs a SELECT on the table with function codes and prepares the menu items in the order in which the SELECT statement returns them to the program. There is no explicit sort on this internal table and hence the result WILL be unpredictable in different systems.
PS:- You could raise a note to SAP asking them to provide the ability to control the positions of these menu buttons and items, similar to how it's done in Dispute Management using Function Profiles.
Thanks,
Edwin