Is what I do in my work, adding an amount to the cost of the laborers without informing the client, considered a permissible commission (ju'l) or brokerage fee (samsara), and must I inform the client of the amount I take and the amount the laborer takes?

Establishing a place for interior design and contracting for permissible benefits is permissible. Undertaking the execution by providing workers and supervising them is a contracting agreement that falls under the category of 'istisna' (manufacturing contract), which is permissible under certain conditions. It is not necessary to inform the client about the details of the work or what is spent on the workers; rather, agreeing on the total price is sufficient.