Still the system. There’s two ways to ring it up, normally, or 2-for-1.
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They can’t do 2-for-1 in your case because it will cause -1 inventory, (and you’ll still just be getting one at full price).
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They don’t have authorization for discounts, so they’d have to involve a manager and nobody wants to do that, (ma fan!)
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So the only option is a regular price ring up.
Sensibly they should remove the sign at this point, but they may not have authority to even do that. 
Not their problem, mei ban fa. 