Is the 1.5% interest per month on delayed payments imposed by the platform considered a usurious contract, and what is the ruling on money earned from advertisements in this case, knowing that I do not delay payments?

The amounts due to the platform for advertising books are considered a debt. A debt is an obligation that is binding upon one's responsibility. It is not permissible to impose a financial penalty for delaying the payment of a debt, as this penalty is considered prohibited interest (riba). The profits you earned from advertising permissible books are lawful for you.