For the bulk export of customers in QuickBooks Online, you need to do following steps :                                                   

  1. Enable bulk customers export settings in basic settings tab.                     
  2. You will get Export to QuickBooks option in customer bulk options in customer listing page.                   
  3. Select customers that you want to export and then select Export to QuickBooks option in bulk actions list.                     
  4. All customers is exported successfully in QuickBooks Online.