Hi,
Please refer to our help page on third party integrations here:
http://www.e-junkie.com/ej/help.integration.htm
That help page describes the format in which we can post your order data to an outside URL, such as Stamps.com or a script you have set up to import that data into Quickbooks.
You can also download your sales data directly by going to Seller Admin > View/Download Transaction Logs, so you should be able to have your order data posted to Stamps.com and manually import your downloaded sales data into Quickbooks.
