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If the 20% fee was not disclosed on the sales agreement (the paperwork you received at the time you ordered your hearing aids), the company cannot enforce this charge.
If you are unsuccessful with working with customer service, you can try the BBB - the BBB is fast, free to consumers, and is often successful (all disputes result in a public report so many businesses work hard to create a positive resolution): bbb.org/consumer-complaints/file-a-complaint/get-started
If you believe that they are charging the amount unfairly, or engaged in unfair business practices, you can report the matter to your state's attorney general, consumer complaint division (the AG does not prosecute individual claims so they will not get your money back for you, but they will investigate and potentially take administrative and/or criminal enforcement action against the vendor).
If the fee is not disclosed in the paperwork you have a "breach of contract" claim. If the fee is disclosed you do not and you are going to have to rely on pressure of customer service and good public relations to get them to refund you this money.
Even if you do have a solid claim, I would recommend the less time consuming and expensive remedies first (an unsuccessful BBB claim does not compromise a small claims action).