channa

I disagree. The benefits do not transfer, it's not a policy, it's a bug.

It works sometimes, which is great for the passenger. But it doesn't always work and cannot be counted on. Using that rationale will weaken the argument.

I would suggest the ONLY rationale to use is that United accepted the 3 bags at checkin, and then denied them at the counter. That will be more difficult for them to contest -- their own website took to the bags, and the agent went rogue and overrode what the system already allowed. Had the site not accepted the bags, you would have packed differently. Make a stink that the agent's job is to only tag the bag and make sure they're within the weight and count specified in the machine, not to impose additional rules after bag fees (if applicable) have been paid.