There is a good reason to pay. It might seem like there isn't, but it's a bit like driving on the right side of the road, or within the speed limit, even when there is no other traffic around. I mean yes you could get away with it but social convention is important.
It reinforces the expectation, the unwritten rules that people ought to tell the truth.
Your actions in not paying undermine this expectation. It has a small impact on social dynamics.
You can say oh, it is only two coats. It doesn't matter. But that's like saying oh, I'm only going to take one of these flowers from the park, I'm only going to drop one bit of litter.
If everyone did that, society would be quite unpleasant. Most people stick to the rules. Some don't, but it is the ones who do, who keep our society as generally decent as it is.
There are two camps, one working for social ease and comfort, the other working, in their own small, selfish, pathetic way, against it.
Every time you do something like this, you are stacking up points on the wrong side.
That's why it matters. Because people should expect that their mistakes are treated as mistakes, not as gifts to the canny shopper. People should expect to be told when they have made an error, and allowances should be made to put things right.
This isn't aimed at the OP as she is past caring clearly but to others who may be interested in the debate, in why some of us care so much about this.
I'd rather be part of the pro-society bunch than the anti-society, anti trust lobby any day. But that's a question of self respect I suppose and if you cannot respect yourself you probably have your own reasons behind that but it is pretty sad.