Change of service level agreement without consultation or notice

Wumdrop changed the rates paid to drivers without prior consultation as stated in the service level agreement thus enabling them to pay drivers a much lower rate as they originally stated. They do not allow drivers to choose deliveries as stated but assign deliveries to drivers thereby costing drivers huge amounts in running costs while they reap the rewards and pay drivers much lower rates. For the work I have done in 2 days I am short paid over R1000.00. They do not care and are not bothered about this behaviour. I wonder if they sleep at night knowing that they behave in such a manner.