Ive actually had the same thing happen with a shipment to Canada.
Twice it came back here. Twice I turned it round with the same pre-paid postage label on it.
The 3rd time it got to the guy, almost 4 months after it was first mailed. I got an email "thanks, the Hard Rock bottle openers arrived!"
At no time during the 4 months did he ask where they were.

On the France business, some European countries utilise a courier network (DFDS?) to carry some of their inbound international packages.
When a package goes with DFDS it doesnt go through customs, they are entrusted to determine the duty themselves, collect it, and pay it over to customs. Supposedly to speed it all up and enable COD duty rather than someone having to pick it up. Simple rules for DFDS - collect the duty or return the package.