Is it permissible to continue working for the postal service, which involves distributing letters from insurance companies, banks, and investment firms, given that more than 70% of the authority's income comes from usurious increases on savings accounts, while leaving some letters undelivered due to the suspicion of usury?

Delivering letters to their recipients, even if from forbidden institutions, does not affect your work unless the letters contain something forbidden, such as an usurious contract. In that case, delivering them is prohibited. However, if they do not contain anything forbidden, then delivering them is not prohibited. It is not permissible to destroy or neglect the letters, as that is a dereliction of duty and may lead to the loss of a right. The mixing of the entity's money with some forbidden elements makes its money mixed, and there is a difference of opinion regarding the permissibility of dealing with mixed money.