Is it permissible to take a commission from a client without his knowledge for mediation between him and a company, or to add this commission to the product's price at the company's request?

There is no objection to taking a commission from the company in exchange for mediating between them and the client, as this falls under permissible brokerage. If the client requests a search for a product, it is not permissible to add a commission except after informing them. However, if the client does not make such a request, and you market the company's products with an agreement that any increase is yours, then it is permissible to sell them at an increased price without informing the client. This is supported by the statement of Ibn Abbas: "There is no harm in saying: Sell this garment, and whatever exceeds such-and-such an amount is yours."