Am I permitted to add an amount to the repair cost of clothes that I send for maintenance to the shop where I work?

It is not permissible for you to add an amount to the cost of repairing clothes or maintaining devices and equipment without the employer's knowledge, as this is deception and betrayal. Allah Almighty says: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]." And the Prophet, peace and blessings be upon him, said: "Whoever deceives us is not of us." You must return what you took without permission to its owner, even by indirect means, for the Prophet, peace and blessings be upon him, said: "The hand is responsible for what it took until it returns it." If these tasks are outside the scope of your official work, you must inform the employer and obtain his approval for the wage, for the Prophet, peace and blessings be upon him, said: "The wealth of a person is not lawful except with his willing consent."