QuickBooks is currently rolling out Bill Pay powered by Melio to QuickBooks Online (QBO) users. You can easily identify your clients that have Bill Pay powered by Melio enabled, because on the "bills" screen in QBO you'll see "schedule payment" buttons. Like this one:
If you don't see this button in your clients QBO file please see this FAQ:
https://melio.zendesk.com/hc/en-us/articles/4418718307228