What is the ruling on subscribing to postal savings services that offer optional profits resulting from the earnings of investment projects for deposited funds, and what is the ruling on benefiting from the service without profits, or taking the profits and spending them on charitable projects if they are Haram?

If the depositor receives a fixed amount for their deposit, it is prohibited interest (riba), regardless of whether the deposit is invested in halal (definition) projects or placed in interest-based banks. However, if the depositor receives a common share, such as one-third or one-fourth, there is no harm, provided that the deposit is invested in halal projects, and that the capital is not guaranteed by the investing party. If the institution guarantees the capital, then it is a loan with prohibited interest according to Islamic law. Therefore, it is not permissible to participate in this service in this form, even if the participant does not take the interest, because good intentions do not make lawful that which is unlawful.