Broker commission covers the work that the broker does on behalf of the insurance company. Broker fee covers the work done on your behalf. Personally I know where I work charges 8.5%.
If you are not happy with the fee they charge, take your business to another broker who charges less. Alternatively you may be happy with the service they provide you in which case you may be more than happy to pay 20%. Have they actually charged you this or is it just in the Terms of Business. They may not have charged that amount but is the max they can charge.