Greetings, all!  I was thrilled to find your group over the weekend as I'm certain you have knowledge I lack.  One element of the feedback I received at London 2022 (LV) was that I needed the same description of rates/routes on more pieces.  With examples from around 80 countries in a display (open) class exhibit that is a fairly daunting task.  Still I have a few French items that are interesting and that seems as good a place to start as any.    I have two items sent months apart in 1873 from London to Nantes. But the individual had moved on and resulted in a redirected cover that is particularly involved. I have identified the Paris.- Bordeaux Convoyeur but looking at a map, the fact that anything going to Nantes (or on to St. Nazaire) went through both of those cities raises more questions than answers.  Let me start the discussion by asking which of the markings present are part of the "normal" routing of mail from London to Nantes during this time?  That would separate out those involved with the redirection.  Any help you may be able to offer would be most appreciated. 

List the postmarks in chronological order, some are indistinct but they appearCalais entry mark ?Paris Bordeaux travelling post office 27 August ?Nantes 27 AugustSt Nazaire 28 August

So Calais to Paris TPO  then part way Paris Bordeaux TPO then to Nantes and then St Nazaire. There is no redirection charge in France. An item has to be handed back to a postman or taken immediately to the post office with the new address. If put back into a post box treated as a new sending. Postage 30c due