As of now, QuickBooks Online (QBO) doesn't support the Point Of Sale (POS) and Customer Relationship Management (CRM) system. You'll want to look for a third-party app for POS and CRM that integrates with QBO. You can browse some apps through the Apps menu. Here how.
In your QBO:
Go to the Apps menu.
Select the Find Apps tab.
Type in CRM and select the app that works best for you.
Click Get App Now.
Select your firm or select a client from the Install for Client drop-down menu.
Follow the onscreen instruction, then click Save & Sync.