You can't just mandate low costs by fiat. That's why it causes market distortion. You could also say mandate that say breakfast at a hotel should cost no more

You can't just mandate low costs by fiat. That's why it causes market distortion. You could also say mandate that say breakfast at a hotel should cost no more than $1. You'd get that, but you'd also pay for it via rising hotel prices that would need to subsidize that benefit some enjoy with a higher room fee that everyone would pay for. In this case lower shipping rates for some are subsidized with higher shipping rates for others, or in some other countries with the same problem corresponding state subsidies for the postal service paid for out of the general state budget.