Not sure how long ago that was, but the current policy is this: If a normally-stocked item sells at one price, and it goes on sale within 30 days at a lower price, AND if the item is still in stock in the warehouse, they will do a Price Adjustment for the difference. Once the item sells out, they won't do the price adjustment. So you need to act fast. You only need to go to the Refund desk and ask for a Price Adjustment. (Depending on the warehouse, they may refer you to the Front End Supervisor's desk by the checkout, to do the adjustment.)
If you find the item has sold for less within 30 days after you bought it, AND it is out of stock (so the Price Adjustment is no longer available), your only option is to return the item for a refund (within 90 days, if it's electronics.) The down side to that is you can't just buy another of the same item, because it's out of stock. So that isn't helping you much, except to refund your money if you felt you should have been able to buy something for less than you spent.
As in all things, the policy is subject to change at any time. Next time you're in the warehouse, stop and ask them what the current policy is, if you think you may want to take advantage of it.
Dave