I have some information to share with linking a shopping cart into QuickBooks Online (QBO).
To let you accept credit card payments, you'll have to link our Merchant Services into QBO. This is the only payment service that we can connect to QBO.
As for your other question, we have a dedicated team who can help you link a web shopping cart into QBO. I suggest posting this question as a new topic in the Intuit Developer Community to get direct assistance from one of our product experts. You can also ask them how to get the production client secret key.
I'm sure they will take care of this for you. I'd appreciate if you can update me on how it goes. I'll be here if there's anything else you need. I'm cheering you to continued prosperity.