What is the ruling on marketing computer software, and is it permissible to deal with companies that offer commissions to the one who first brings the customer, or to the last one who brings him, or both, and what is the ruling on dealing with companies that set a period for the commission to be due?

It is permissible to act as an intermediary for lawful goods in exchange for a known fee, and this falls under the category of brokerage. There is no objection to specifying the conditions for the commission, such as it being for the first or last person to guide the customer, or that the commission becomes due if the customer makes a purchase within a specified period. If the website does not explicitly state it, it is more likely that the commission is for the last person, especially if there is a long delay between the first person's guidance and the purchase.