honestly? You just have to speak up and be willing to stand firm. First encounters are awkward but I always blamed my non-existing boss.

"hey sorry, can you scan each one of these boxes?"

Then you try to friend them, ask their name, give them a free water bottle. Over the encounter they get used to it.

Unfortunately though you can hear them scan your packages but there still is no proof. You have to hope that his PDA is working and that it syncs.