I can't quite agree with you either, because it's true, every employer is looking to get cheap labor and the cheaper, the dumber (no offense), but in the same time, imagine this woman has to do this every single day, several times. If one time goes wrong, next time she can start with a smaller quantity and add more, till she gets to the quantity needed. I mean you can learn on the go and do better next time.
Another thing that is a no brainer as I said, you don't have to be a rocket scientist to know, blaming the customer is the worst thing you can do in this business, especially if what happened is your fault. These people could evolve and make progress every day, but for some reason they chose not to.
Anyway, congrats to you for what you have achieved in your field of work and I'm sorry to read that you got fired for asking for a raise, especially so close to retirement. Some people have no heart and your boss is one of them. We live in a cruel world.
Thanks for the valuable comment!