Is the opening fee legal?

Actually, it's a check-in fee. The "opening fee" you mentioned should be the initial installation fee of some user facilities, which is charged by those service providers, not real estate developers. This is also the default rule at present, and the default is also legal. For example, if there is no gas in your current house, you should apply for installation. If you don't want to pay the initial installation fee, people won't install it for you. And the commercial housing has been installed, you can't say that I don't want gas, I don't pay, I burn wood, and you tear down the gas. So before you move in, people ask you to pay a similar fee as one of the conditions for completing the check-in procedure. So, there is nothing to remedy, just relax.