I have tried QBO on Firefox and Chrome as well as Incognito mode in both with no luck.

I did try to resolve the problem through the chat tech support and after an hour and a half the answer I got was

"I really apologize for taking so long. Upon consultation regarding the issue, there might be an update that
Quickbooks has made. Currently no option to bring back the flat rate. A product limitation. However you may
send a feedback regarding this. I’m truly sorry to hear about your experience"