I want to make sure you're getting the best help with payment gateway integration for QBO.
I'd love to provide you the SDK for the QBO payment option in Android and IOS. However, we have a dedicated team who can help you more with your concern. I suggest posting this question as a new topic in the Intuit Developer Community so you'll get direct assistance from one of our product experts.
If you have other questions about QuickBooks Online, please let me know. I'll be around to help. Have a great day!