Do you not plan to go grocery shopping or do you always do it on the spur of the moment? I'm not sure what the issue is. If you need to pick up something small from the store on the way somewhere you can definetly get a small always carry on you bag that will fit in a pocket. When you're going to actual go grocery shopping just bring the bigger reusable bags. If the purpose of the journey is shopping it's not inconvenient to carry those bags and you'll have to carry the grocies back anyhow.

I get that it's less conventient to have to remember a bag but it's not some insurmountable task and it does seem to reduce the amount of plastic bags that get caught by the wind and blow around as trash.