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Divide the total by the number of units before you reach for that second bottle of shampoo: the label says “2nd unit 70% off” and the real discount is 35%. One bottle costs 4.90 €; two, at full price, 9.80 €. With the offer you pay 4.90 for the first and 1.47 for the second: 6.37 € in all, which is 3.43 € less, and on 9.80 that comes to exactly 35%.
The rule holds for the whole family: the real discount is half the advertised one. “2nd unit 50% off” is 25%. Three for the price of two is 33%.
And that percentage only exists if you were going to buy two. If one was enough, your discount is zero: you have spent 1.47 € on something you did not need.
Check the starting price as well. Since 2022 the crossed-out price has to be the lowest that product has had over the previous thirty days; if it turned up last week, the sum does not begin where you think.
The only figure worth comparing is the unit price you end up with: 6.37 divided by two is 3.19 € a bottle. Write it on your phone before deciding and set it against the same shampoo with no offer on the next shelf: if it is under 3.19 €, the offer was not an offer.