What is the ruling on reducing the taxes imposed on shop owners in exchange for receiving a sum of money from them?

There is no sin in evading illegitimate taxes and assisting shop owners in doing so, because it falls under the category of removing oppression. As for taking compensation for this assistance, if you expended effort or money, then it is permissible for you to take fair compensation (اجرة المثل); otherwise, no. All of this applies to illegitimate taxes. As for legitimate taxes, it is not permissible to scheme against them.