France's competition authority, the Autorité de la Concurrence, has fined 20 package delivery firms €672m (£488m) for colluding to artificially raise prices annually, reports the BBC.
The UK's Royal Mail was among those fined for its part in the cartel.
It will pay a reduced fine of €55.2m, down from €67.6m, because it agreed not to contest the fine.
The price rises, which happened between 2004-10, were agreed during "secret meetings", the watchdog said.
The Royal Mail's French division, General Logistics Systems (GLS), accounts for about 17% of Royal Mail's revenues.
"Royal Mail recognises the absolute need to comply with European and national competition law and the necessity to prevent infringements," the firm said in a statement.
"It has implemented an enhanced compliance programme in GLS France in order to strengthen the culture of competition law compliance."
The companies fined include the French divisions of FedEx Express, TNT and DHL Express. TLF, the delivery union, was also among those fined.