We are now working on a Quickbooks integration project. The client has a storefront and all sales are stored on a database. Their Quickbooks, however, is the desktop version, not the online version.
So there is no easy way for those sales data to be transferred from the web server to the QB desktop application.
We are building for them a small custom system, using the QuickBooks PHP DevKit. This system will transfer all customer data, and all sales data, from their online database, to their Quickbooks desktop.