The Export Value has not much to do with exporting the file.
The Export Value is the actual field value you get with
this.getField("myComboBox").value
in Acrobat JavaScript.
When you print, you see the Face Value.
You may think of the Face Value being what is of interest for the user, and the Export Value (aka Return Value) being what is of interest for the form and/or the back end process attached to the form.
A very common use of Face Value and Export Value is to select a staff member and then to display more information in other fields. Face Value is the name of the staff member, and the Export Value is the index of the entry for that particular staff member in an internal table, which contains the additional information.
Now, if you want to print the Export Value (but keep displaying the Face Value), you could put a text field over the combo box, and set it to hidden on screen but visible in print. You then would add this line of code to the Calculate event of that cover field:
event.value = this.getField("myComboBox").value ;
Of course, you will have to enter the actual name of the combobox instead of "myComboBox"…