Sales tax or use tax?

we are in the granite and cabinetry business in Chicago, we offer delivery and installation of our products. We import most of our goods from overseas, does use tax apply to these goods? Someone informed me that when a customer purchases cabinets from us, along with installation, the customer is exampted from sales tax, and if the customer only wants the cabinets, then sales tax will be applied. Is that true? Please help!!