I’ve been a fast-food manager before, baltotop, and while you never specifically stated the kind of business you were in (except that it involved stocking, customers “checking out,” and making coffee), I can guess that it’s a similar experience.
Unfortunately, in this case, I think the customer may be right. If you are covering a register for someone else—for that person’s break, perhaps, or while that person takes an emergency phone call—you aren’t granted any special rights or privileges when dealing with the public. You don’t get any slack from the customers because you were “just covering.” Your customers have no way to know what your particular excuse was (“I was just covering, this is really my day off but I was called in, this is my first day, I’m from a different store but I’m working at this one today because they were short-handed, my power went out at home so I don’t have clean laundry, my iron is broken so I look rumpled, I ran into a doorknob in the dark last night so I have a black eye, our store was three people short this morning so we’re way behind right now”) and it’s not their job to know. I mean, you wouldn’t want to buy a substandard pair of shoes because the regular shoe-making guy was sick and the manager was “just covering,” or buy a lemon of an automobile because the union was on strike and the scab workers were “just covering.”
Covering a register was frowned upon at my place of work because it made it more difficult to track accountability for cash shortages (because the thieving employee could always steal and blame it on you, or the thieving manager could always dip into the till and discipline the employee for it). However, if I had to cover a register for any length of time, it was my responsibilty to make sure I was fully prepared to be nailed to that spot until the person was back. If that means I took two minutes to straighten my tie, run to the restroom, grab a Coke, make a phone call, gobble down a sandwich, or whatever, then I’d do that first before agreeing to cover. I’d take the position as seriously as if my boss had stationed me there.
I can understand being frustrated with a customer’s attitude. If the customer was truly that irate about so simple a matter, then I agree it would have been an overreaction. There’s no excuse to be foul to another human being for something so minor. Still, perhaps you ought to think about ways you can prevent being absent from a position you consented to cover.
A former manager of mine put it best. An employee was asked to cover a morning shift, but the employee wandered in an hour late. He seemed to think that because he was “just covering,” he didn’t have to be on time or work to acceptable standards, as if merely showing up to collect a check were a tremendous favor to us. My manager at the time said, “I don’t care if you’re covering Jesus Christ’s shift, you’ve still got to do the job.”