If, after all of your efforts, you still need a plastic shopping bag and paper is not an option: YOU STILL CAN LIVE WITHOUT ONE. DON’T DO IT! IT’S A TRICK!
But if you’re going to take a bag no matter what I say… Let’s have some fun with it. Open someone’s mind today without being obtrusive.
Here’s what you do:
USE LESS, and as little as possible. You can do it! Refresh your mind on creative ways to NOT use a bag in THIS post.
Most Important: Make a pact to recycle the bag. It’s your moral duty as a bag user. Or reuse it in many many many ways. Recycling is highly preferred. Take bags and incredible other plastic films to your local grocery store to be recycled! Read on.
It’s my opinion that recycling plastic bags is the greenest option, unless you otherwise purchase and use plastic bag waste basket liners anyways. Then you should replace them with used bags from stores when you do take them.
Save money and manufacturing! To me, in this case it is greenest to replace those otherwise purchased plastic waste can bags with used grocery bags or produce bags if you are using them anyways. SAVE MANY BAGS from the landfill this way long term.
As far as the nature of the interaction, the same silly possibilities apply here as when you ask for no bag.
1) Completely inconsequential
2) Enlightening to pleasant
3) A bit like you are supervising
4) Downright awkward
Ask the clerk to put as many items in a single bag as possible. These clerks can be extremely gun shy. Whatever your bag is made of, encourage them gently to put even more in the bag than they are comfortable with. 
The clerk may continue to be unsure, and so they may hem and haw a bit; they really don’t want the bag to rip on you etc… Stick to your guns and they will follow up! You are teaching them trust. This is good 😉 This falls into the “a bit like you are supervising” interaction category.
Then keep an eye on this process as bagging is finishing up to make sure they don’t double bag it in the end anyways!
Happens all the time- they are nervous and/or trying to help. CLASSIC face palm moment. 
If you hadn’t noticed, double bagging will also happen frequently if you don’t watch the process. A little micro management is often warranted here- you can pretty much guarantee it 😉
So, clearly these kind clerks are only trying to help, really. They are afraid your groceries are going to be smashed to smithereens. They don’t want to kill your wife. It’s so sweet and strange. Be nice and be sure to explain your WHY. Tell them that you will be super careful, and not hold them responsible.
Then actually be super careful… them bags are made to rip! Be realistic and speak up for yourself, as with life in general.
There was actually a major plastic bag lawsuit in 2011. Walmart was sued for not double bagging! There was a toe infection and a death… Oh, my… pretty severe, but seriously up for debate!
The moral is this: USE YOUR POWERS OF INTELLIGENCE. That can always be put in all capitol letters, haha! I cannot be held responsible for bag or no-bag related injuries. Or any green your life related injuries for that matter. I trust you and your intelligence. You will hear it from me again: stay green, and stay smart! 😉
In drastic or appropriate moments I have been so bold as to say something like this, with LOTS of sympathy in my voice:
“ I just know too much. Many animals die in the sea because of bags. It’s really, really sad. I just can’t do it. Thanks though!” It might leave it just a tad awkward, but I am walking out at that point, so whatever!
Now they are thinking, and I didn’t accuse them of anything. Stating fact without blame. Certainly, this is not a needed quip in every situation. But whip it out in drastic or appropriate moments 😉
Plan to do some variation of this dance with every clerk forever. This is so that you won’t be annoyed in 5 years when this is still happening to you! It’s just another amusing- albeit frustrating- game to me.
Visualize yourself: The Superhero. This Earthling knows little of the problem, and knows nothing of your mission. Cut them some slack, and empower them with knowledge. Invite them to check out this blog!
Like I said, even if it ends awkwardly, I walk away smiling. It feels right. I done good.
Tonight, I will walk away smiling as well…
Here is a fun quote for the road:
Dan Smith on Quora, speaking about packing your own grocery bags for efficiency:
“…you approach the checkout and find that there’s some kind of charity fundraising thing going on and the local Cub Scouts are going to pack your f* bags for you and then expect a cash donation after they’ve ruined your life.”
My sentiments exactly 😉
