Hello there, Carpow.
Allow me to share a brief explanation on why Standard users doesn't have a Customize option at the bottom of the invoices in QuickBooks Online (QBO).
If you're a Standard user, your access to some QuickBooks features is limited. One of the limitations covers your ability to customize invoice templates.
To know more about the different user types and levels of access in QuickBooks Online, here's an article that you can refer to. This discusses the complete list of what a Standard user can do and can't do: Learn about user types and permissions in QuickBooks Online.
Know that I'm always around here in the Community if you have further questions about user permissions. I'd be glad to clear things up. Have a good one.