When I stand patiently at the counter don't come over to me and scream when no one is around " CAN I HELP YOU?" Then when I tell you I want a sub and you tell me there are some in the cooler don't get snarky or pissy when i tell you no there aren't, I checked. All in a calm/polite manner.
Then have your co-worker come help you and say within earshot of me and an elderly man waiting for his meat to be sliced "I need some help, there are no subs in the cooler and those two people are staring at me".
We were standing at the counter waiting to be served. If you don't like your job or waiting on customers go find a different job, I didn't know that being polite when ordering something should make me feel like I've inconvenienced your life oh so much.