From
https://www.audax-club-parisien.com/en/our-organizations/brm-world/#rulesArticle 10
Overall time limits vary for each brevet according to the distance. These are: (in hours and minutes, HH:MM) 13:30 for 200 KM, 20:00 for 300 KM, 27:00 for 400 KM, 40:00 for 600 KM, and 75:00 for 1000 KM.
Additionally, riders must arrive at each checkpoint between the opening and closing time for the checkpoint and calculated as follows:
Opening: 34 km / h (km 1 to 200); 32 km / h (km 201 to 400); 30 km / h (km 401 to 600); 28 km / h (km 601 to 1000); commercial rounded by the minute.
Closing: 1 hour + 20 km / h (km 1 to 60); 15 km / h (km 61 to 600); 11.428 km / h (km 601 to 1000); commercial rounded by the minute.
These times are noted on the brevet card with the information for the checkpoints.
Riders must arrive at each checkpoint between the start and the finish while the checkpoint is open. If a rider arrives at a checkpoint after it has closed and the ride organizer is satisfied that the rider’s lateness is due to the occurrence of something unforeseen and beyond the control of the rider (such as stopping to help at a traffic accident or a road closure), then the ride organizer may waive the fact that the rider arrived at the control late and allow the rider to continue. Poor bicycle or equipment maintenance, fatigue, lack of fitness, hunger, etc. are not unforeseen and beyond the control of the rider and therefore will not serve as a valid reason for being late.
Subject to the foregoing, failure to make all checkpoints, even if the brevet is done within the overall time limit, will result in disqualification.=====
The postcard thing is self-certifying that you posted the postcard between when the control opened and closed. Deliberately arriving after the control has closed is quite different to that.