Is it permissible to accept payment for intercession (shafa'ah), especially if it is not stipulated as a condition?

"Using one's influence to achieve a benefit for others does not justify taking compensation, unless the person of influence provided a service that merits compensation. It is unlawful to take a fee if it was stipulated at the time of the contract. However, if the beneficiary volunteers something to the person of influence, there is no harm in that."