This society pays lip service to accessibility, it’s not thought through, it’s not something the average person considers. The mere idea of an invisible disability probably didn’t come up at that store from design to implementation of their new layout to their employee training.
I can only imagine how terribly exhausting that must have been for you and how disheartening. Can you find the energy to write to the management of Safeway and explain your experience? It seems to me there is a law that states someone has to help a disabled person if that person needs help to check out. I believe here in California a clerk has to help or accommodate a disabled person.
I would try writing corporate management or emailing them with a friend link to this. Include the location of the store you went to. Who knows? They may even send you a gift certificate to compensate you for your suffering. They ought to. I’ve had surprises like that in response to complaints. At least they will probably express displeasure to the store manager. And they might explain how to get the help you need if you ever have to go back.