Easy, don't put that notice in every box, and don't expect that everyone that receives the notice would bother filling it out, and maybe once you are winding do

Easy, don't put that notice in every box, and don't expect that everyone that receives the notice would bother filling it out, and maybe once you are winding down that product, start delaying or just not sending the gift cards. You only need to keep ahead of the other items in the same product category in terms of average score and review count. That's public information, so it's pretty clear how many of these gift cards you need to give out to top the category.