In order to push Estimates and Purchase Orders to QuickBooks, each Item in the SI 2015 Project must be linked to an Item in QuickBooks. You can create new QuickBooks Items directly from SI 2015 or you can map Items in SI 2015 to existing QuickBooks items. Once Items are "linked" you can also synchronize data changes for the Items between SI 2015 and QuickBooks.
The following links provide additional information on the topics covered in these videos:
Creating/Linking QuickBooks Items