Yes, sgmken, stamps.com is only applicable for QuickBooks Desktop.
You'll want to check our third-party apps that can be integrated with QuickBooks Online that have the same functions with stamps.com. Please click here so you'll be routed to our Apps Center. You can filter the apps to display according to its category.
When you click the app, you'll see its Overview and Reviews of other customers on how it helped their business.
You can always post your questions here if you need anything else.