Take Advantage of Lower Price Guarantees!
A few weeks ago, I purchased a clearance item at Home Depot for $30.00. There were only 2 units left, so I thought I had better pick up one before they were all gone. Sure enough, a week later, they got 6 more units in and marked everything down to $20.00. So, I went to the customer service desk and asked them to paper return my previous purchase and resell it to me for $20.00. No complaints. They just returned the item and resold it to me for $20.00, saving myself another $10.00. Kudos to Home Depot on that transaction.
Not all stores are like that, however. I purchased an item at Menards and it went on sale after 30 days. That is beyond their normal price guarantee period. I went to the customer service desk and asked them to give me the lower price, having the original receipt in hand. At first, the representative told me that it was too late for the price guarantee and there was nothing he could do. I knew better.
I asked him how long I had to return the original purchase, and he confirmed to me that I had 90 days to return it. I agreed. Then I asked him if the store wanted me to buy a new item on sale and return the old one so I could get the lower price. He was confused. I asked him to ask a manager before I bought a new item at the lower price. The manager came over, understood what I was going to do, and told the young rep just to paper return the purchase and resell it to me at the lower price.
If they would have told me that they could not reduce the price, I would have just returned the previous purchase and let them deal with an open box item. They came out ahead by giving me the lower price and not having a return to deal with.

Menards is a good store and I buy lots of stuff from them. In fact, I even use their store Credit Card, which I get 2% cash back on purchases. I don't know if using the store credit card bought me any goodwill in that specific transaction, but at least they know that I am a regular customer. They make lots of money on me every year.
Point is, you think you got a good deal on a sale item and then find out a better price later. You might have options to get the lower price on a previous purchase if you ask, even after the normal price guarantee period. But you have to be willing to advocate for yourself to save money. I guess that's just a part of being frugal, knowing how to play their game!