Hi jb20147,

You don't need to upgrade to be able to disable the custom numbers feature. The said functionality can be turned off by going to the Gear icon. then selecting Account and settings. Go to the Sales tab. Then, toggle the Custom transaction numbers button to Off.

If it's already turned off, yet, the system still reverts it automatically, I suggest performing some troubleshooting steps to isolate the issue. To start with, let's use an incognito or a private window: 

• Ctrl + Shift + N  for Google Chrome
• Ctrl + Shift + P  for Firefox
• Control + Option +P for Safari

When the setting sticks, go back to the regular browser and clear the cache. After clearing, close out any browser instances to complete the process. We can also use another supported browser as an alternative. 

If the issue persists, I recommend reaching out to our phone support team so we can investigate this further. You can do so by clicking the ? Help icon.

Moreover, I've added here an article in case you want to send customer statements: Create and send customer statements in QuickBooks Online.

I'm always a post away whenever you encounter system issues.