I'll share the steps with you on how to link a third-party app in QuickBooks Online, james-braund.
Let's go to the Apps menu and check if iZettle is one if the supported apps that can be integrated in the program. Here's how:
- Go to the Apps menu and proceed to the Find Apps tab.
- Type in the name of the app in the Search bar and press Enter.
- You can also click the drop-down list for Browse category and select a category.
You can also check out this article in case you need to disconnect an app from the App Center: Disconnect An App.
Just in case iZettle is not a supported app, you can still import your customer payments and banking transactions as a CCV file. Let me share this article with you as a guide: Import Bank Transactions Using Excel CSV Files.
The Community is always here if you need anything else.