Is it permissible to use an advertising platform that demands additional fees for late payment, given that this condition is not actually enforced, and my personal experience and the support team confirm that it is not imposed, and that the platform merely closes the advertising account in case of delay, and there is no available alternative to this platform?

It is impermissible to impose a fine for late payment of advertising fees, which are considered a debt owed by the advertiser. This constitutes the usury (riba) of pre-Islamic times, which is forbidden. It is also impermissible to enter into a contract that includes such a fine, due to the prohibition of engaging in invalid contracts. However, as long as this clause is practically nullified, and the contracting party does not adhere to it, and it is neither mentioned in the contract nor requires a signature, there is no harm in requesting and utilizing the service. Nevertheless, officials should be advised to completely remove this clause from the website.