I’ve purchased a lot of plugins in the last few years and I can spot a well coded, optimised one.
The UPS Access Point plugin by the guys at Renoovo Design is brilliant; I’ve had no conflicts with any other plugins and it works exactly as promised.
The one issue i did have during set up was server side which their technical support solved for me with a blistering turn-around time.
More recently I had an Issue with the UPS API, Clément kept me up to date and provided me a quickly written patch for the issue.
Couldn’t recommend more highly.