How do I tell my Costs/Expenses not to Combine on the Invoice?

If the Bank Reconciliation code is set up with a GL offset account, then when you use it in bank reconciliation, the period and year fields will be accessible.

Otherwise if the code is one that does not create a transaction in GL then the year and the period will be greyed out.

You can check the transaction codes that are setup by going to AP Manager / Tools / Transaction codes.  If you enter a GL offset account when you set up the code, the GL account will be hit when you use the code.  If you leave the GL Offset Account field blank, then no GL entry will be created when using code.