Is it permissible to buy a money box with the intention of making it an ongoing charity (Sadaqah Jariyah), and to place a message inside for the family to donate its contents after one's death?

Bequesting money as charity is permissible, and it is an act of obedience for which the doer will be rewarded. It is executed unless it exceeds one-third of the testator's wealth. If it exceeds one-third, it is not enforceable without the heirs' consent. However, this charity is not considered ongoing charity (ṣadaqah jāriyah); ongoing charity is an endowment (waqf), meaning the dedication of a specific item and giving its usufruct as charity. If you wish to have ongoing charity, invest the money in something that can endure and whose benefits can be utilized, such as a well or a fruit-bearing tree, and dedicate it to the poor and needy, or stipulate this in your will.