Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The transaction which subtracts 15% for cashing the cheque in advance of its time is forbidden, as it contains a form of Riba (interest) which is giving a loan for a period of time in exchange of an additional interest. So the business lends the amount of the cheque to the customer but subtracts 15% of the amount for the delay of the period in which the business gets the total amount.
But if the 2% percentage is purely for administrative expenses which are pre-determined, and it is not for cashing the cheque in advance of its time, then there is nothing wrong about this. This is considered as a permissible charge.
Allah knows best.