Before Invoices, Payments, or Supplier Invoices (aka Purchases) can be exported, they must be Posted.  Effectively, Posting means finalizing a document so that it can no longer be edited.  It is important to note that Invoices and Payments are Posted separately.
 
There are two circumstances in which Invoices may be Posted.  If you do not have terms with any of your Customers, and each Invoice is paid at the time of the transaction, you can check the “Post Invoices When Paid” option in the Company setup panel to have Invoices post when paid.  If you do accept credit terms with your Customers, and you accept payments to Invoices after the transaction has been completed, you can Post all your Invoices, Payments, and Supplier Invoices over a specified date range.
  
To Post your Invoices, Payments, and Supplier Invoices, choose the Post By Date option of the Edit menu. To Unpost an Invoice, open the Invoice and choose “Unpost Invoice” from the Edit menu.