Is it permissible to take a commission for marketing goods that are not purchased until the customer opens the letter of credit?

What those who wish to market for commissions do is called brokerage (simsarah), which is mediation between two parties to a contract to finalize it or to guide to it. Brokerage is permissible as long as it does not involve assisting in the sale of something prohibited or guiding to it. It falls under the category of ju'alah (commissions/reward for a task), and the principle of ju'alah is that the worker does not benefit until the work is completed.