Is it permissible to buy from a website that offers free goods, where the buyer pays a percentage of the item's price to the site administration, knowing that the administration makes the arrival of its share at the buyer's discretion and cannot guarantee it?

What the website does in providing means of communication between the seller and the buyer falls under the category of permissible brokerage. The website is entitled to take a percentage of the price of sales exceeding one thousand Saudi Riyals. The buyer must fulfill this condition, due to the generality of Allah Almighty's saying: (O YOU WHO HAVE BELIEVED, FULFILL [ALL] CONTRACTS), and the Prophet's (peace be upon him) saying: (Muslims are bound by their conditions).