the sku and item description indicates a third-party sold the item via bestbuy’s web site. it was upgraded by that third party before final sale. the extra ram would be covered by that entity, not bestbuy.
but we do not know if bestbuy actually returned the modules that were originally in the laptop, nor do we know if it was the oem one or the added one that was ‘bad’. we also don’t know if the third-party seller simply added one so-dimm to the original configuration or pulled the original to install a matched pair instead.
if it was the oem one, it should have been covered by lenovo’s warranty, i.e. replaced by bestbuy as a warranty repair–the laptop having been upgraded is irrelevant to that remedy.