How does the invoicing work?

Before you create a new invoice, make sure you set the proper default invoice details at Start (in case of Blue Aero theme)  > Configuration > Network Invoice Format > Invoice format  while you choose "WYSIWYG" view.

To generate (create) an invoice in Post Affiliate Network you have to navigate to  Start (in case of Blue Aero theme) > Accounts > Account manager > click edit (pencil icon) next to any account > Accounting (tab)    and click the  Create invoice button.
When you click that button a window will show up as shown below:

Let's explain some fields from above:
Date from/to:  the date range of the so far not paid (billed) approved commissions of the particular merchant. The Date to is always one day back from the date when you click the 'Create invoice' button.
Due date: date by which the invoice has to be paid.

Commissions: all the commissions that are approved and so far not paid (billed) (commissions earned by the particular merchant's campaigns) till 1 day before the day of creating the invoice.
E.g.: if you are creating the invoice on January 31st, then it will include commissions earned till January 30th.  

Fixed = a fixed fee the merchant agreed to pay (this fee shall be also mentioned in your terms&conditons in order to play fair)
Percentage of commission = here you can define how many percentage of the merchant's commission has to be paid to you by the particular merchant.
Amount:  the total amount of the money (Fee + Commissions) the particular merchant has to pay you.

Balance after this: This equals to the "so far not paid Amount" - "Commissions"

When the merchant pays the invoice, you have to mark it as 'paid' by pressing the dollar icon  next to the particular invoice in the Accounting (tab)  section.

In the Accounting section you see also a Balance  field, which is calculated globally as "Payments" - "Commission".