saleing with mediator to customers

Assuming the tax invoice is issued by you to your customer directly, the consideration can be received through other person. For collecting money and paying to you he might charge some commission. Therefore, he may be proviiding some services to you to that extent (you can call them as collection agent). There is no bar in appointing someone to collect money on behalf of you. Ensure that the tax invoice is issued by you to the customer directly.