+1 Make that part of the deal. We only need 2 items, but you will produce 4. We will then randomly pick 2 of them for testing, and then and only then if those 2 pass will we actually use the remaining 2 as needed.
This happens all the time in construction of multistory buildings. When the floors of concrete are poured, they also produce smaller sections of concrete from the same batches. As the concrete cures, it's strength is tested. This ensures that the concrete can support the weight designated in the plans. Better to watch a test pour fail than the building collapsing when cheap concrete fails under load.
[edit] oops, just read comments later in the thread stating the same thing
This happens all the time in construction of multistory buildings. When the floors of concrete are poured, they also produce smaller sections of concrete from the same batches. As the concrete cures, it's strength is tested. This ensures that the concrete can support the weight designated in the plans. Better to watch a test pour fail than the building collapsing when cheap concrete fails under load.
[edit] oops, just read comments later in the thread stating the same thing