I think you did just fine, except for feeling guilty about insisting on civilized behavior.
Taking a tangent, this seemingly ingrained response to be polite and feel guilty if you can't assuage the other person's "anger" even if they are being a donkey's rear is exploited by thugs and sociopaths (overlapping categories) to manipulate you and put/keep you at a disadvantage.
The non-violent ones will try to manipulate you into giving them some resource, whether it's spare change, buying a pencil/roof/job/car/investment you don't need, or an undeserved place in line.
The violent ones will use your over-trained politeness and compassion to get close enough to you to overwhelm you with violence or threat of violence.
Because in general we have actually done a pretty good job at keeping the two-legged wolves penned up in certain areas/environments and don't suffer from
widespread groups of marauding vandals, a lot of people have forgotten that the basis of law and civilization is violence on the behalf of the rules of society. Thus we get beliefs that any kind of violence is bad, which is a perfect environment for bullies and thugs to exploit. Which they do.
Being civilized does not mean being a schmuck or a pushover or never using force/violence. It does mean defending the civilization via word and deed, and occasionally that deed might be shooting some clown that is trying to take your or someone else's life. That's why self-defense is an inherent human right, and why the 2A is important in protecting that.
You defended civilized behavior via word, and I think you did exactly right (except maybe turning your back on her - see below). While you noticed that the others in the line were also annoyed with her, notice that none of them did anything and you were standing out there by yourself. Congratulations. You now know what it's like to be a sheepdog.
It's true that not every transgression is worth doing battle over, and often times people are unintentionally rude or things just get out of sync; we need to choose our battles wisely.
This woman was intentionally rude. You were not rude. You were a real civilized citizen.
You do have to keep in mind that when you defend via deed, you may have to back that up with defending by force, however. Some sociopathic types don't know when quit:
http://www.freep.com/article/20130324/N ... on-Detroit" onclick="window.open(this.href);return false;
A great book to read to help sort out what is going on when somebody acts out, whether on the street or in the office, is this one:
ConCom: Conflict Communication A New Paradigm in Conscious Communication
This could replace most of the management and communication books out there.
It's only $8.99, and only in Kindle format. Kindle readers for your computer and phone are free. If you don't like electronic formats, make an exception this time, this one is worth it. Read it, then replay that scene at the WalMart and see if it doesn't explain what was going on there.
Best wishes.
p.s. The ConCom book by Rory Miller linked above should be used to replace the entire de-escalation portion of the CHL training. If I were a CHL instructor, I would strongly recommend this book and Miller's other book
Facing Violence: Preparing for the Unexpected