Richard Watterson and I have a similar problem...
I am conflicted whenever I go to the grocery store. I like using the self-checkout and bagging my own groceries, using my own bags that I bring and thereby cutting down on the use of plastic. As a matter of fact, a nearby city (Long Beach, CA) has actually banned plastic shopping bags, and my own city is considering the same measure.
However, the stores seem to want you to use their own reusable bags and no others. If I put any other bag other than the ones with their name on it, I get the same message Richard Watterson does below (from the episode "The End" in The Amazing World of Gumball).
(Click for Richard trying to use the self-checkout...)
There's usually a button that says "I'm using my own bags" but when I press that it usually locks the whole system up, and one of the store people has to come over and enter a code before I can continue.
IT SHOULDN'T MATTER WHERE THE BAG COMES FROM TO PUT YOUR DAMN GROCERIES IN IT!
I suppose I could avoid all this and wait in line fo ra regular checker, but I kind of like doing it myself (less the above headache). Plus I kind of get a kick out of using a Trader Joe's bag at Albertson's. ;p