Or do they never let you know what they plan to discontinue and one day you show up and want to buy it and it's gone. Label is gone. Something else is in its place?
Downsizing product is a manufacture ploy to boost their profits. Stores have no control over this. P&G started the downsizing fad (Pampers), other companies jumped on board when they saw that it increased their bottom line
Some companies will raise prices on products to put them on sale. For instance, a tube of toothpaste had regular price of .99, but the "sale" price is 10/$10. Bib tags are deceptive as well. People assume when they see a bib tag below a product that the product is on sale. Close examination will reveal that this is not always the case. End Caps are another ploy. Just because a product is on an end cap (or in a shipper) does not mean its on sale.
Over 70% of customers "grab and go". Time is a valuable commodity to people and stores know this. Store schematics are designed to "manipulate" people into buying more than they came in for. (why do you think milk is always located at the back of a store?) By placing certain items near other items (tie-ins), teh store can boost sales and still get their customers in and out quickly.
Then there's the subliminal messages stores use. Ever smelled bread cooking inside a store? How about BBQ chicken? Makes one hungry for them. Ever seen racks of "hot, fresh" french bread by the checkout? Stores also have employees make announcements (around lunch and dinner time) saying how succulent a certain product is. Advertisements on the in-store radio" are at a slightly higher volume than the "canned-music".
Deception (illusion) is used to sell everything from chewing gum to houses. The human brain is wired to "assume" things (that way it can process things faster). To the brain, bigger always means more. A taller tree is bigger than a shorter one. A bigger rock has more mass than a smaller one. A bigger freeway will move cars faster than a 2-lane road. Sounds good, but if one thinks about it these aren't necessarily true. Companies spend billions of dollars each year deceiving people so they will buy their product ... and its perfectly legal. The name of a product; the shape and color of packaging; advertising (words on the product) ... these (and more) are used to "deceive" (persuade) the consumer to buy.
Some people think they they are smarter than stores. They think they can spot their "tricks". Trust me, even the most savviest shopper isn't immune to the "tricks of the trade". I know a lot of them and yet I still fall for them.