The product codes for ordered items are not shown on the Invoices/Order Report produced in ProSelect software.

However they are included in the xml file produced when you export orders such as with "Standard XML" or "Standard XML for web" (it depends on which studio management database you use, this information may or may not be imported by them).  Product codes are also displayed in Studio Cloud when using "Export Orders to StudioCloud' (in ProSelect v2014r1 and newer).