Greetings, tsd81.
I'm here to provide some insight regarding this matter.
I've managed to replicate this on my end, and I got the same result. After some research, I discovered we don't currently have an option to disable the billable checkbox.
It's my priority that you're able to use QuickBooks seamlessly, so I'll be sending feedback based on this interaction. The updates and changes to QuickBooks are largely the results of feedback provided by users, just like yourself. That said, I'm making it my top priority to submit feedback to our engineers and make them understand the importance of an option to disable this.
In the meantime, I would recommend that you manually uncheck them for now when you create the bills. Please don't hesitate to let me know if there's anything else I can help you with. I'll be here to help you out.
