Please ensure that you have registered your application to integrate with QuickBooks Online (QBO) so that your clients can import the bills and print the checks in OBO seamlessly.
If you're more interested in API, you can check out with our developers. They can provide you this data. I would encourage you to visit their Community website. Then, click the QuickBooks Online tile and the Ask a question button. From there, they can answer your concerns about third-party integrations.
This site is exclusive for software developer enthusiasts who want to increase QBO flexibility towards other applications.
Additionally, you can check the QuickBooks Apps Center anytime to study how other applications integrate with QuickBooks Online.
Feel free to place a comment if you have other questions. I'll keep my notifications open. Stay safe and well!