Is it permissible for a commercial establishment to request 25% or 30% of its commission in advance for preparing the documents of a fish import project, and when is the commission rightfully due according to Islamic law?

What you mentioned about commercial mediation falls under the category of brokerage (simsarah), and brokerage is considered a type of ju'alah (commission/reward for a specific task). Many scholars hold that stipulating a cash payment invalidates it. However, voluntarily offering to pay all or part of the fee is permissible. So, if your client volunteers to pay the fee you request, there is no objection. But stipulating it as a condition in the contract makes it invalid. The commission is due after the completion of the work, and offering it voluntarily in advance is permissible.