Is it permissible to take a commission by increasing the price to the client without the knowledge of the printing press owner who is doing the printing?

There is no impediment to taking payment for printing designs, provided that a legitimate Sharia contract, such as an agency or brokerage contract, is concluded with the client. This contract should clarify that you are an agent or intermediary between him and the printer, and that your agreed-upon fee is permissible (halal). It is permissible to agree in advance on the cost of the design and printing. You are not required to inform the printer of what you agree upon with clients, unless there is an agency contract between you. It is forbidden for an agent to inform his principal of an unreal price to take the difference for himself, because that is deception.