Rather Lacking

The company I work for is VERY big on customer service. We rely heavily on an extremely loyal customer base and so we go out of our way for them every day.
Sometimes this is particular situations like the woman I had in a few weeks ago whose husband had ruined his favourite trousers by getting a biiiig stain on them while doing some DIY. We had no advice on ways to remove it (it was not coming out) so instead managed to track down a suitable patch (the trousers were no longer current stock) which involved cutting up a spare pair found at head office...
Other times, it is just our policy that gives the good service. We have no 28 day limit for refunds or exchanges. We have one of (if not the) longest guarantee in the business on waterproofing. If we start a 20% off promo and people come back in complaining they hadn't heard one week, two weeks, three weeks ago when they bought their x, y, z, we refund the difference in gift vouchers... Sometimes I get irritated because we are a small company and would people really do that to M&S or Debenhams when they have one of their blue cross sales? Maybe some would but I suspect not as many as do to us. Anyway. The point is, I spend my time at work bending over backwards for people (sometimes I'm happy to, sometimes it grates, it rather depends on the person) and so I have very little tolerance for bad service.
This week, I got very cross with myself for not saying to someone how utterly abysmal their service had been. Instead I left. Then I spent the rest of the evening fuming and plotting what I might do to rectify the situation while knowing that I wouldn't be bothered to do any of these things.
I was in a shop on the way home from work. I was looking to buy a few things that I required but I was buying treat versions rather than cheap versions. There were signs all over the shop saying that if I spent £20 I would get a free bag. As I I was spending nearly £20 on what I wanted I thought I might as well tip it over the £20 mark as a free bag never goes astray with me. Several sales assistants checked if I needed any help and one of them pointed out to me that there was an offer still on that ended that evening so she'd removed the signs. She did not say "so the offer that all the signs in the shop are for is not running yet". Nothing of the sort.
When I got to the till and made my purchase (which included getting sucked into a discount card thingy) it became apparent that my free bag was not forth coming. However my money had been taken by then and I was tired. It twigged that if one offer was still running the one advertised everywhere wasn't. I stated this much. The woman (who was the manager making this even worse) simply said "no" at the end. I was tired. I was grumpy. I just walked out. I did not point out how poor her service was. I did not point out that in my shop if we put the signs up early so we can go home on time, we honour them. It is our fault, not the customers that we are over-keen.
Now I'm left just not wanting to ever go into that shop again. But I like their products. Grrrrrrrrrrrrrrrrr.