I believe this to be wrong, the seller should follow the LAW in Iowa, which is here:
Specifically:
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423.3 Exemptions.
There is exempted from the provisions of this subchapter and from the computation of the amount of tax imposed by it the following:
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90. The sales price from the sale of solar energy equipment. For purposes of this subsection, “solar energy equipment” means equipment that is primarily used to collect and convert incident solar radiation into thermal, mechanical, or electrical energy or equipment that is primarily used to transform such converted solar energy to a storage point or to a point of use.
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Three is NO REQUIREMENT for the buyer to have an exemption certificate or file ANY form of ANY kind. The exemption of sales tax is exclusively controlled by the TYPE of thing being sold, not the buyer.
If a vendor insists on charging you sales/use tax for on solar equipment delivered to Iowa, they are NOT following the law. Indeed, it is ILLEGAL to charge sales tax on solar goods shipped to Iowa.
Vendors find it easier to charge you sales tax than to get their systems right. Their convenience is costing you money.
Mike C.