(In Germany) The legal purchase agreement is made at the register, which means you agree to those prices. The prices on the shelves are technically irrelevant, although if they are intentionally falsified you could sue for deceit or false advertising.
Which is why almost all stores will honour the prices on the shelves, even if they’re wrong, and also it’s just cheaper to adjust the price than to argue with customers ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯