The Advance Commission is charged by the freight forwarder for paying the customs duties & taxes upfront to the customs authorities.
For more context, the customs authorities will not release the goods at the port/airport before the customs duties & taxes are paid. To avoid any delays in the process due to the payment that could result in extra charges (demurrage, detentions, and other storage/processing fees), forwarders are using their own customs deferment accounts for these charges and thus take on financial risk (in case the buyer cannot pay or anything goes wrong in the declaration process).
For this financial risk, it is market standard to charge 2,5% on the total customs duties and tax value.