Customer does not return to pick up their framed artwork after one year
Does your store have a written policy that answers this question? Even if it does, have you tried to contact the customer? Do that, and do it in writing using a provable method of delivery, and give them a definite time to pick up their art or you'll no longer be obligated to store their art. Imagine them suing you, and plan for your defense by showing how reasonable you're being before you get rid of anything.
To make sure this doesn't keep happening, provide your customers with appropriate and lawyer-drafted written policies about payment and pick up requirements.
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