Regarding the "messy account passing", I'd just like to be able to interact with a vendor's own UPS account with minimal setup from my vendor. Thanks for the info! It clears some things up.
I don't know a service that offers multiple courier services and allows multiple accounts... If you want you can integrate the API of one courier service, such as UPS, and offer access with multiple accounts. Or you can use Easypost and have access to multiple courier services with one single account. I'm sorry that I couldn't be more helpful. Good luck!