The branch that I've submitted works for now at RC2, but it looks like we might be running into some changes to the underlying API here: https://www.drupal.org/project/commerce_ups/issues/3387353 ✨ OAuth 2.0 support? (D10+) Fixed . I've got this solution set up in production and have yet to run into an issue.
Once there's an accepted solution for that, I'll try to reroll these changes to work with whatever the new API layer is.
This is a critical issue for me, so I'll be keeping an eye on this.
Okay - I rerolled this and it seems to be working correctly now. Changes from #18 and #19, rebased to 3.x-rc2.
Combined and updated the patches from #18 and #19 to the tip of the latest dev branch. This is roughly around 3.x-rc2.