Retailers who ask customers if they would like to donate to charity after a sale are using it as a test to see if they can raise prices.
Do you know this, or are you assuming this? It is certainly possible that, at the margin, someone is willing to spend money on a charity but not spend that same money on the product. Although I suppose there's probably a correlation between donating and willingness to pay more.
It’s a way to test shoppers and evaluate their willingness to add a marginal dollar for spending. That can be correlated against buying behavior and taking advantage of discounts.
Right now, for a grocery store, that means they can put butter on sale that women will buy for Christmas cookie making, and market sprinkles and decorative sugar at higher prices (or with a coupon) to increase customer value. That MDA heart that you bought on Labor Day weekend helps assess the effectiveness of that sort of promotion.