If you have a UPS account (free to create), you can browse the Marketplace Shipping section from the Shipping tab on the UPS.com homepage.
See screenshots below for a walk through. Please reach out again if you have any questions.
The screenshot below illustrates the actual connection page where sellers enter their login credentials. We prepopulate 4 of the 6 fields, but if users have customized their FTP path, for example, the can overwrite it here.