Is it permissible for someone who buys goods online for others to add an amount to the original price without informing the buyer, in exchange for the effort and time spent in the purchasing and delivery process?

If someone appoints you as an agent to purchase an item, it is not permissible for you to increase its price without their knowledge, as this constitutes deception and betrayal. It is obligatory to inform them of the actual price, and you are not entitled to take anything without their knowledge. However, it is permissible to agree with them on a known fee in exchange for searching for the item and bringing it.