Dude, if you're sick, call in sick.

Sure people lie about it, but its also a situation where someone could have caught food poisoning or a cold could have gone from "eh, stuffy nose" to "I'm vomiting and feeble". In either case, letting the person take the day off helps ensure it won't repeat again the next day. After all, if they are indeed sick and come in, now they're recovery is impeded and they may be sick longer, plus they probably were borderline useless that day anyway. If it does repeat to the point where its a serious issue, then they can be asked to get a doctor's note. That indicates whether its a chronic issue that needs accommodated, or if they've been lying they'll be in the next day.

Having been in retail before, I absolutely get being pissed that someone's called in sick *again*. It means more work for the people who are there, and it can't be scheduled or planned for. Even so, the real issue is always that whoever has been doing the scheduling and hiring is putting y'all on a skeleton screw staff. One person being absent should never be crippling to everyone else. If it is, then either someone else needs hired who can perform their tasks as well, or there's flat-out too little manpower available at the job to deal with unexpected events. Either one is a failure of the business/manager/scheduler not a failure of the absent person.