In order to fully utilize the post month feature in Beanworks, please make sure that

Netsuite is configured to allow posting into other periods.

In Netsuite go to Setup -> Accounting -> Accounting Preferences -> General Ledger section.

Make sure you have enabled the function "Allow Transaction Date Outside of Posting Period". Ensure this field is set to “Allow” or “Warn".

Enabling this will allow invoices to be imported from Beanworks into Netsuite with the desired posting period. 

To learn how to enable and use post month in Beanworks, please read here.