Twice I’ve ordered things through Amazon.fr, strimmer cord and saxophone reeds, and they’ve arrived posted from the UK. The vendors covered the extra postage.
Liam you made no comment so I assume you missed this amongst the forum noise
Is the diatomaceous earth you use food grade…??? If so then I use it too both as a dietary supplement and if necessary in the coats of my Border Collies and added to their raw food to guard against internal and external parasites…and haven’t yet found a cheaper alternative here…I get mine from wiggly wigglers in UK…x 
Just wanted to mention that John Withall’s suggestion to try the phrase “produits chimiques” worked well for me. I don’t search using Google; I use DuckDuckGo’s search engine called DisconnectMe (https://search.disconnect.me/). It’s much more private. I found Onatera’s website (https://www.onatera.com/), where I found Epsom Salts which I’d been searching for to no avail in local shops and supermarkets in my area. I confess I didn’t try searching for the other chemicals Liam was asking about.
Thanks, John! And many thanks Liam for bringing up this topic, so I could find something I’ve been looking for, for over a year. Cheers!
For those looking for epsom salts, Action often have them, as well as a lot of other chemical things in their hardware section.
Looks like you will be able to get hold of neo-nicotinoids fairly easily.