Cadbury chocolate company was founded by Quaker John Cadbury in London in 1824. Tea, coffee, cocoa and drinking chocolate were not only delicious, they were also considered to be healthy alternatives to alcohol, which Quakers deemed bad for society (many, but not all, still do).
Early Quaker merchants initiated the fixed price system, believing that bargaining over the cost of merchandise could lead to untruthfulness. This was, in part, as part of our testimony of integrity). In order to avoid the temptation to exaggerate the worth of their goods, and to avoid encouraging customers to do the opposite, they set what they figured was a fair price and stuck to it. Customers liked the system, and it took hold generally.
For more info, check out https://www.cadbury.co.uk/the-story