A very polite and appreciated practice common in the Upper South i.e. southern Ohio and Indiana, West Virginia and Kentucky. It's more commonly done at stores that offer double coupon redemption such as Kroger and Meijer than stores that don't such as Walmart.
When a store has a super-double coupons redemption day or week, it is even more practiced.
"I had more toothpaste coupons than I could use, so I droplifted them before they expired."