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One thing they tell you in Vietnam is that you never get anything for free. The people who say that have never been to a typical Vietnamese supermarket.

Lipton Tea Buy the small box of tea bags (20 count) and get a handy-dandy drinking glass!

Dutch Lady Dry Whole Milk Powder (the normal way to buy milk here is either UHT or powdered--whole pasteurized/homogenized milk usually gets bad getting it home) Buy a small box and get a genuine faux crystal serving bowl, shaped like an apple. Great for cold cereal.

Kellogg's Frosties (Frosted Flakes in the U.S.) Get a one liter bottle of UHT milk with the large box.

Nestea Sugar Free Ice Tea Mix Buy the small package and get another handy-dandy drinking glass. It's a little smaller than the Lipton glass.

Colgate Toothpaste Buy the large tube and get a travel tube of toothpaste free.

Nutifood Dry Whole Milk Powder Buy the medium sized box and get a free 1 kilogram bag of sugar.

Head and Shoulders Shampoo Buy a big bottle and get a free tube of Crest toothpaste (normal size).

Nestea Sugar-Sweetened Ice Tea Mix with Lemon Buy the small package and get a pretty shaker cup.

Campina Dry Whole Milk Buy a large can and get a child's jigsaw puzzle.

Rexona Antiperspirant Deodorant Buy either the roll-on or stick and get a Zodiac sign necklace.

Rexona Antiperspirant Deodorant (discontinued offer) Buy either the roll-on or stick and get a colorful pair of socks (will fit a child or a typical Vietnamese woman).

That´s normal procedure with manufacturers who want to make money.
More often than not, it will be cheaper for a manufacturer to give extra material rather than a price discount.
That´s the deal with the "Jumbo-sized" chocolate bars or "supersize meals" at McD´s, too. An extra 20% of pop or some 20 grammes of fries will cost A LOT less (in production costs) than the extra quarter the suck- errrm, customers will be glad to pay.