Is work that relies on adding a specific percentage commission (such as 20%) to a service bill provided by a first party to a client (second party) considered a type of prohibited financial transaction or usury, given that the client is aware of this percentage beforehand, and taking into account that for some services, a fixed amount is added instead of a percentage?

If the company knows that you are merely intermediaries, and that the concluded contract is a brokerage contract, not a sales contract, then there is no harm in this work; it is a permissible form of brokerage and is considered a type of ju'alah (commission/stipend). If the percentage taken is from a known amount, there is no problem with it, nor with a fixed amount. However, if the price of the service is unknown at the time of the contract, then the ju'alah cannot be a percentage; rather, it must be a specified amount. The person from whom the commission is taken must be aware of it. If it is taken from the buyer, then he must be informed of it, and it is not permissible to charge it to him as if it were part of the price without his knowledge.