The ZheroTax business model is to not charge a fee unless the tax professional reduces or eliminates the taxpayer’s amounts owed. Either the taxpayer wins or they don’t pay. However, if they win, there is still a fee and the people at Tax Pages have done a great job of explaining why those fees are generally deductible from net income tax. Read their wonderfully technical and yet still accessible explanation for why.
In addition, read the CRA’s own publication on the deductibility of legal and accounting fees.