I have always been taught that one can buy necessities on Sunday. Toilet paper, milk, water, or something like that.
When I was a child, this was not a difficult question simply because NOTHING was open other than one gas station that sold toilet paper, milk, bread, and dog food.
That was it. There was also a restaurant or two that was opened Sunday afternoon.
But generally speaking, everyone knew you better get what you needed on Saturday or you would do without on Sunday.
As a general rule, we don't shop on Sunday except for stopping for milk which is $2 cheaper at the grocery on our way home from Mass.