Is it permissible to pay membership fees to a political movement without knowing how they are spent, and feeling embarrassed not to pay, even though it is optional?

The ruling on paying these fees depends on their purpose. If their purpose is permissible and they are spent on legally permissible benefits, then there is no harm in it. However, if these fees are paid for forbidden matters or to achieve unlawful goals, then it is not permissible to pay them.