My husband who has dementia reads most of the time. He can read fine but does not comprehend or understand what he is reading. I try to explain things to him about this but he ends up getting frustrated. He thinks I am the one who does not understand the topic. He will look at the same papers for hours and hours at a time.
Pick your battles. Reading without comprehending seems to me to be harmless To keep their loved ones occupied many people give them towels to fold, taking some from the linen closet if there are none fresh from the laundry. Reading the same thing over and over without understanding it seems to me to be on a par with folding the same towels over and over.
Ordering things on the phone has a higher potential for a negative outcome. It is a good thing he can't usually complete such calls himself. When you take over, disconnect his call or dial the weather number if he insists that you call, and then place the order and say, "Oh, you are out of that item? When will it be in stock? OK we'll try again then." This is geneally easier than convincing him that he doesn't need/can't afford the item. This particular behavior may drop off on its own. I think my husband's phone period lasted a couple of months.
If you keep in mind that arguing, reasoning, and explaining really don't work with someone who is losing his ability to reason that may reduce your frustration. You cannot fix the problem. The goal of making every one less miserable is a good one!
Why point out to him that he isn't understanding it, when that only makes him frustrated? What is your goal?