What is the ruling of Islamic law on the "Post Guarantee" service, where a subscriber pays a monthly sum to receive a larger sum after a period, and what is the legitimate alternative if it is unlawful?

A postal savings account or postal guarantee is a type of forbidden deposit, as a known percentage is deducted for the owner of the deposited amount and added to the principal, and this is precisely usury (riba). The Shariah-compliant alternatives for this transaction are Mudarabah (definition), regulated by Shariah controls, and cooperative Takaful insurance.