Is it permissible to take a percentage from the factory after the sale without a prior agreement, and must the merchant be informed of the actual shipping cost if it is less than what is customary?

Mediation between a seller and a buyer (brokering) is permissible and falls under the category of ju'alah (commission/reward). It is a condition that the fee be known. The majority of scholars prohibit the fee from being a percentage. It is permissible to take the brokerage fee from both parties or from one party, according to the agreement. If the other contracting party pays the broker money without a prior agreement, it is considered a gift. As for taking a fee for exporting goods, if you inform the client that the agreed-upon export fee is what shipping companies charge, then this is forbidden because it is a lie. You must inform them of the truth, and you are allowed to take a known fee for your agency. However, if you agree with the client to export the goods for a specific amount, regardless of the shipping companies' costs, then you are allowed to take what was agreed upon.