Are commissions taken for mediating between a seller and a buyer of oil barrels, and documenting that in the contract, Islamically permissible, even if these commissions are large and most transactions were conducted via the internet and phone?

Brokerage is permissible as long as it does not involve assisting in a forbidden sale. It falls under the category of a commission (ju'l), and the validity of the transaction is not affected by whether the commission is large or whether the broker did not exert much effort for it.