UCC 2-601 in the US universal commercial code. This is often called the Perfect Tender Rule. This law cuts both ways. A seller is required to provide a product perfectly conforming to the order to trigger the buyer’s obligation to accept and pay.
However if the seller does not meet their obligation under the perfect tender rule, the buyer has the right to one of three resolutions: they can reject the entire order (even if some parts do conform), they can accept part of it and reject the rest and make appropriate payment, or (relevant here) they can accept the shipment in full and make the payment. This completes the contract and transfers ownership of the shipment as if the perfect tender expectation was met.
Further, most states have laws meant to protect against predatory mail order sellers that can apply here, the product received could be argued as an unsolicited upgrade, which in many states is legally free - the seller cannot force you to pay for or relinquish such an item.
