I have the complete opposite logic. I have to drive my gas guzzling suv to get Kleenex and a few other items OR the person delivering Amazon packages can drive a few minutes from the house down the street that is getting an Amazon package.
Amazon is not driving the Kleenex from the warehouse to my house in an empty car!
My delivery is an incremental carbon footprint. Going to the supermarket is significantly worse.
The person said to buy it the next time you happened to be at the supermarket. Not to order a few items at a time. I believe they were specifically talking about going out of your way to be environmentally conscious.
Write yourself a note if necessary (post-it, phone app, whatever) and buy Kleenex the next time you're at the supermarket.